How about ajax submit

Currently also looking for this functionality. It would make sense since we already moved the form to a block we can assume the form is located on a page with loaded content, which most likely doesn't need reloading.

This could be done in conjunction with Ajax submit, http://drupal.org/project/ajaxsubmit.

Related issues from the AJAX issue queue:
#361992: Make UI for JS hook for form submission (about making a user interface widget that toggles redirection after submit)
#360545: Error with Form Block module (here is described a way to enable this functionality in a custom module)

edit: I just realized nedjo was referring to the AJAX submit module, while my links are issues of the Ajax module (aka AJAX Forms) (which provides similar functionality, though, and is therefore relevant as well.)

The issue is now fixed in the dev release of AJAX (see the issues above).
I have successfully tested this solution. -> Marking as fixed.

One more testee confirming Ajax (w/ disable redirect) + Form Block works like a charm.

Just a quickie primer for anyone doing this:

1. download formblock and ajax modules
2. enable formblock, ajax, ajax_ui, & ajax_disable_redirect
3. edit the content type you wish to use and enable formblock
4. configure the block to appear on the desired page
5. configure the ajax settings (enable ajax, enable disable redirect option)
6. optional: if adding the formblock to a view, use ajax_views_refresh to refresh the view (requires #823478: Doesnt work with latest Ajax Dev)

This is why I love drupal-- AMAZINGLY simple to do for such a complex page ui.

Bravo to all the module maintainers!