I've been beta testing with a single -> multisite, a single site and a wpmu -> multisite. No major issues, just some weird 'This is what happens when you use a beta/RC version.' Nothing that couldn't be fixed in 5 minutes.

Demetris - The problem with a known issues release is ... unless a beta tester had the problem, there's no way to know. 10k plugins, and MAYBE 200 testers? We'd each have to use 50 totally different plugins to have a chance of it :( As it was, I know of a handful of plugins we did find as being borked, and many of us betas did tell them and work out some solutions :)

Caryn Goldsmith - Open a topic on the WP.org support forums if you haven't already.