Seeing how Young Hahn, David Rothstein, Paul Lovvik, Charlie Gordon, Daniel F. Kudwien, myself and others work on parts of the Drupal 7 user experience implementation already, it is getting harder to apply all the patches, keep up with their development and get them integrated to work together with various efforts.
So to ease our work, I've sat down and set up a third party code repository for the D7UX effort. Using a third party repository for core development is not anything new. For example, the Fields API work used a third party repository and the Drupal 6 internationalization work also did that. When multiple people work on projects which need integration and multiple patches to apply at once, it is easier to experiment in a third party repository and roll core patches from there. Once core patches are accepted or declined, and code freeze comes, we stop using this repository, since it's purpose is going to be over.
The D7UX repository is hosted on Google Code at http://code.google.com/p/d7ux/ (thanks for Google's support for Open Source development) and is already used by Young Hahn and myself with others to hopefully follow. You can check out the work anonymously based on instructions from http://code.google.com/p/d7ux/source/checkout
Currently, it hosts the header patch created by Young and two alternative versions for the overlay implementation by myself (blog post coming up). It also includes a D7UX install profile for testing. Our work is planned to be integrated in Drupal core itself, so this install profile is a temporary measure to keep the number of our core patches low, but let people quickly try out our work. We manage a list of the patches we applied at http://code.google.com/p/d7ux/source/browse/trunk/patches/patches.txt
Both Young and myself are administrators of the repository, and we are open to collaborators, so reach to us if you'd like to collaborate.