Have you tried AT&T's GSM service? It's usually a bit more expensive than T-Mobile, but they use GSM850 which has much better coverage (especially if you have coverage problems when you go indoors), and in CA they've got free roaming onto cingular which is the same network as T-Mobile so you've got the best of both worlds.

With Cingular's AT&T buyout, Cingular is keeping the AT&T network in CA and selling their own network to T-Mobile. So eventually the roaming won't be quite as nice as it is now, but at the moment it's great.

Matthew