On my installation, superseeding the builtin widgets doesn’t work. I added the Categories-Widget to my Sidebar, but the builtin one still shows up.
This would actually not be an issue, if the builtin widgets would offer support for internationalization – which the non-hardcoded Categories-Widget does.
See http://www.delta-phi.net for an example. Switching to pixeled-de is no longer a viable option as I am trying to run a multi-language blog.
Judging by Google results, more people have this problem. If you have some information on what needs to be done I’d be happy to help out. Im new to WordPress but I’ve done something similar to Drupal – can’t be that hard 😉