I'm sticking with vb 3.X - I've heard absolutely miserable things about 4.0, especially about what it does to server load and the pain in the ass with customizations and upgrading from previous version. I'll wait until it is more established.
I wish Automattic would just buy vBulletin, update it to this millenium, and fold it into their current offerings. I know they have a BB system but theirs kinda sucks in comparison, especially on the admin side of things.