Bamboo 4.0 will support branch-by-branch workflows in hg/git - the developer can 'fork' the mainline branch into his/her personal branch, work, and ask Bamboo to act as gatekeeper between personal branch and the mainline branch.
However, some people might want to work by forking the whole repo, commit to that personal repo, and ask Bamboo to act as gatekeeper between the personal repo and the mainline repo. This use-case is more probable in environments where strong policies are enforced (like for example casual developer has no 'push' access to the mainline repo, nor the admin access to the Bamboo).
note, that it should be possible to achieve the branch-by-fork workflow utilizing Bamboo 4.0 facilities, but it wouldn't be so straight out-of-the-box, and would force the casual developer has admin access to the Bamboo admin. A few things that we should keep in mind if we implement this story:
* automatic detection of forks (probably not doable outside of Github & Bitbucket)
* merging the same branch, but on two different repositoryUrls