I think my preference at the moment would be to stick with CVS for now (we've got a lot on our plate and it's going to stack higher once we release and start doing some of the tasks we've been discussing in the Design forum).
If subversion's coming out in May I'd probably like to give it a try because they'll have a compelling self-interest in getting the migration tools to work.
If it were to look like they're way off in the future with a meaningful release, we could look into Perforce or Bitkeeper. I'm not so concerned about the fact that they're not Open Source, just free, more with an assumption that Subsersive is more likely to provide an easier migration path and their docs surely will be pointed towards the existing CVS user (part of an easier migration path!)
Of course I could be all wet ...
