I have a Tmobile blackberry, that I just replaced because I never got the blackberry service with cingular. (phone was unlocked). My friend has a blackberry with Tmobile with just the unlimited data plan and he is able to use his phone no problem, so I'm not 100% sure u need the blackberry plan.
T-Mobile Sidekicks are nice if you are a young woman. But a guy using a Sidekick just looks really weird.
Not really...guys can use Sidekicks without looking like a kid. =) I can't really vouch for the Sidekicks because they don't have many features that a Blackberry has but it is wonderful if you are an IM addict.