Import fails: non-stdClass object in $_SESSION['pp_import']

Oh my god, this bug cost me hours and nerves:

Importing a correct panel page, when I hit submit on the general settings page (first step after import), I'm seeing three warnings:

    * warning: Invalid argument supplied for foreach() in C:\xampp\htdocs\drupal5\sites\all\modules\panels\includes\plugins.inc on line 583.
    * user warning: Duplicate entry '0' for key 1 query: INSERT INTO panels_display (did, layout, layout_settings, panel_settings, cache, title, hide_title) VALUES (0, '', 'N;', 'N;', 'N;', '', 0) in C:\xampp\htdocs\drupal5\includes\database.mysqli.inc on line 151.
    * warning: Invalid argument supplied for foreach() in C:\xampp\htdocs\drupal5\sites\all\modules\panels\includes\plugins.inc on line 583.

This also means that the display has not been correctly saved. Debugging shows that panels_save_display() is called with an empty $display object. What's wrong here???
A dprint_r($_SESSION['pp_import']) gives me the following dump:

.
.
.
    [display] => __PHP_Incomplete_Class Object
        (
            [__PHP_Incomplete_Class_Name] => panels_display
            [args] => Array
                (
                )

            [content] => Array
                (
                )

            [panels] => Array
                (
                )

            [incoming_content] => 
            [css_id] => 
            [context] => Array
                (
                    [argument_term_3] => __PHP_Incomplete_Class Object
                        (
                            [__PHP_Incomplete_Class_Name] => panels_context
                            [type] => term
                            [data] => 
                            [title] => Unknown context
                            [page_title] => 
                            [identifier] => Taxonomy term 3
                            [argument] => 
                            [keyword] => term_2
                            [plugin] => term
                        )

                    [argument_nid_1] => __PHP_Incomplete_Class Object
                        (
                            [__PHP_Incomplete_Class_Name] => panels_context
                            [type] => node
                            [data] => 
                            [title] => Unknown context
                            [page_title] => 
                            [identifier] => Node ID
                            [argument] => 
                            [keyword] => node
                            [plugin] => node
                        )

                    [context_node_1] => __PHP_Incomplete_Class Object
                        (
                            [__PHP_Incomplete_Class_Name] => panels_context
                            [type] => node
                            [data] => stdClass Object
                                (
                                    [nid] => 2
                                    [vid] => 2
                                    [type] => story
                                    [status] => 1
                                    [created] => 1211226950
                                    [changed] => 1211226964
                                    [comment] => 2
                                    [promote] => 1
                                    [sticky] => 0
                                    [revision_timestamp] => 1211226964
                                    [title] => Teststory
                                    [body] => 
                                    [teaser] => 
                                    [log] => 
                                    [format] => 1
                                    [uid] => 1
                                    [name] => admin
                                    [picture] => 
                                    [data] => a:0:{}
                                    [last_comment_timestamp] => 1211226964
                                    [last_comment_name] => 
                                    [comment_count] => 0
                                    [taxonomy] => Array
                                        (
                                        )

                                    [files] => Array
                                        (
                                        )

                                )

                            [title] => Teststory
                            [page_title] => 
                            [identifier] => Node
                            [argument] => 2
                            [keyword] => node_1
                            [plugin] => node
                        )

                )

            [title] => 
            [hide_title] => 0
            [did] => new
            [name] => 
            [layout] => onecol
            [layout_settings] => 
            [panel_settings] => 
        )

    [displays] => Array
        (
            [argument_4-default] => Array
                (
                    [title] => Taxonomy term 3 Default
                    [argument_id] => 4
                    [display] => __PHP_Incomplete_Class Object
                        (
                            [__PHP_Incomplete_Class_Name] => panels_display
                            [args] => Array
                                (
                                )

                            [content] => Array
                                (
                                )

                            [panels] => Array
                                (
                                )

                            [incoming_content] => 
                            [css_id] => 
                            [context] => Array
                                (
                                )

                            [title] => 
                            [hide_title] => 0
                            [did] => new
                            [name] => 
                            [layout] => 
                            [layout_settings] => 
                            [panel_settings] => 
                        )

                )

        )
.
.
.

Some research on the subject (see comments on http://de3.php.net/unserialize) revealed that while a session is active, we cannot add custom class objects to the session variable, as classes need to be initialized before the session is started. That's why we are getting this "__PHP_Incomplete_Class".

The common workaround is: serialize object before storing to the session variable and unserialize it after retrieving.
I did this for panels_page and panels_mini, and voilà, everything works correctly now.

I'm curious though whether this bug appears on both PHP4 and 5, and when it has been introduced, as the import functionality hasn't always been broken.