As of 0.15, Bazaar will have initial support for nested trees. Nested trees basically allow you to add a 'magic' directory to a Bazaar branch that in itself is another Bazaar branch. A very good use case of this is if you have a project for which you need to include some library (e.g. popt). Nested trees allow you to add a reference to the upstream popt branch, avoiding the need to synchronize every time upstream fixes a bug.
Subversion has similar functionality, although the term used there is 'externals'. Externals are quite easy to use - using them only requires setting the svn:externals property.
I've done some early work on supporting externals in bzr-svn and mapping them to nested trees. At the moment, bzr-svn is able to track nested trees that are being added. Removes and changes to existing nested tree locations are still on my todo list, but will definitely be supported in bzr-svn 0.4.0.Go Top