Since I haven't seen anyone else mention this... there is also:
http://en.wikipedia.org/wiki/SVK
I say stick with Subversion/SVK:
#1. *everyone* knows how to use it and almost all OSS software
projects use it.
#2. The subversion developers are *awesome*. I used to work with them
at CollabNet and know them personally. Very smart great guys.
#3. SVK solves the problems that you are talking about.
#4. It would allow you to host your source code on code.google.com
for free and be able to have the benefits of taking advantage of
google's infrastructure.
Branches in Bazaar are cheap, unlike in Subversion.
I'm really sorry Barry, but that's just plain untrue. Read this:
http://svnbook.red-bean.com/en/1.0/ch04s02.html "Cheap Copies"
jon stevens http://subetha.tigris.org/