BuddyPress.org

I have used Media Temple for WPMU sites in the past and the Grid is better than most shared hosts, but really WPMU itself can grow past resources quickly. That is why VPS or dedicated is recommended, though many still say they are fine on a shared host if their bandwidth and userbase is low. Other great VPS providers include Slicehost.com and Futurehosting.biz

Getting recommendations for hosts is covered extensively in the WPMU forums, so you might want to pop by for a search there.

Trent