Xenforo is still in beta though, and they're being sued by Internet Brands [webmasterworld.com]. The future there is not certain despite the many fans they have garnered. They might be an option sometime in the future, but not in 2010.
Also, Xenforo doesn't have the Blogs and CMS that vB4 has built in. This will prevent many from migrating from current working vB installs.