This seems like a really nice feature! At work I maintain an open source project but also have a private fork containing customizations just for our centre. This should make it very very easy to keep my private repo in sync with the public version.
BitBucket has been pushing a lot of nice features lately!
I think on the whole, github and BitBucket are very refreshing. They both come out with great ideas, and it fosters a very, very healthy space in which to provide services to users (source code hosting, versioning, and management).
It is also a space that every developer should become intimate with, and I think it's great that it's been brought to the forefront of software development like it has, especially for the visibility it has to new developers. When I started my bachelors in CS 5 years ago, I didn't have source control until my 2nd/3rd year, now it's something at the forefront of learning to program.
BitBucket has been pushing a lot of nice features lately!