Review: T-Mobile MDA

Generally speaking, if this is not something you are able and comfortable with doing by yourself, it's highly advisable that you not do it at all. And yes, it will void your warranty. The moment you bring it back in to a shop to get it "fixed" and they see the new flashed ROM on it, they have every right to tell you "Sorry, Charlie." Maybe they'll be nice and help you anyway, but it's pretty unlikely.

I tend to be a bit conservative and somewhat of a purist when it comes to stuff like this, but in general I would prefer a supported/warrantied device that is a little buggy than an unsupported/non-warrantied one that "seems to work for the time being."