Can't enable upload module

When I go to admin/build/modules, Upload is listed at the bottom of the "Core - optional" list but there is no checkbox to allow me to enable it.

- I've tried this on two different computers, one a Mac and one running CentOS, PHP 5 on both.
- Both are clean installs of 6.0 RC2.
- I've checked the permissions of sites/default/files and verified that they are world writable.
- I did a print_r in system_modules of the form around line 750 and verified that upload was in the form. It is:

                    [upload] => stdClass Object
                        (
                            [filename] => modules/upload/upload.module
                            [basename] => upload.module
                            [name] => upload
                            [type] => module
                            [status] => 0
                            [throttle] => 0
                            [schema_version] => -1
                            [old_filename] => modules/upload/upload.module
                            [info] => Array
                                (
                                    [name] => Upload
                                    [description] => Allows users to upload and attach files to content.
                                    [package] => Core - optional
                                    [version] => 6.0-rc2
                                    [core] => 6.x
                                    [project] => drupal
                                    [datestamp] => 1200005704
                                    [dependencies] => Array
                                        (
                                        )

                                    [dependents] => Array
                                        (
                                        )

                                    [php] => 4.3.5
                                )

                        )

- I've renamed the upload module, directory, and install files "uploader" and then a checkbox *does* show up. Is something else altering the form and pulling it out?!? I'm flummoxed.