Wireless phone advice

Recently, I’ve looked into purchasing wireless phone service. Preferring an Ericsson T68i, I’m researching GSM options.

So far, I’ve found T-Mobile and AT&T plans that come with promotions allowing for a free T68i (The normal cost is roughly $300). If I were to convince Cingular to add it to an existing plan, I’d need to buy one and unlock it. This could be a possibility, if T-Mobile and AT&T don’t pan out, but right now I’m looking at either T-Mobile or AT&T. Which of those two have better coverage and service?

Some general questions, too:

What is the current state of GSM coverage in the US? Concentrating on the northeast portion of the US, what’s the probability I’ll encounter an area with no GSM coverage whatsoever?

What carrier has the strongest GSM coverage overall, especially in the Philadelphia area? Also, do all the carriers have roaming contracts with the others?