For no-contract, there's MetroPCS, but they're CDMA. I'm just looking for a GSM provider so I can use a Series 60 phone.
T-Mobile's coverage is spotty in N. Cal. but they do have no (long-term*) contract options.
*Technically, there's always a contract, it's just month-to-month.
Doesn't AT&T also offer a month-to-month when you bring your own device?
Yes. Take your GSM phone to a *real* AT&T store and you can get any of their plans without a contract. I suggest a real store because alot of AT&T dealers will tell you that this is not an option, but it is.