Forum OpenACS Q&A: Response to Getting ready for beta - tagging and branching CVS
Subversion has the advantage of upward compatibility with CVS in many regards. From reading about it, casual users (those who grab sources or only make commits) may not notice any difference at all, which would be great. The big difference seems to be for those who administer branches and tags and all that. It's been rationalized and improved in that area.