BuddyPress.org

I just did quite a bit of research on this and signed up with WiredTree today. Check WHT for a great coupon that turns their basic VPS (~$50/month) into their top VPS feature-wise for the same price.

Also good options: HostMySite, PowerVPS, SurpassHosting, HostV.

Steer clear of MediaTemple for a WP/MU/BP install. They’re great overall (fantastic support), but the GridService setup they have isn’t particularly friendly for these projects. DreamHost isn’t a high service option, you get what you pay for.