If you want to use a GSM phone, there are two choices in the US; AT&T and T-Mobile. T-Mobile seems to be more reasonable than AT&T, but has far less coverage.
Reasons you might want a GSM phone: so you can travel to Europe (or many other places) and just stick in a prepaid SIM card. So you can switch between carriers without throwing out your phone (though at the moment this is more theoretical than practical, since AT&T and T-Mobile use different 3G frequencies, and most phones don't support both; you could switch, but would be back on EDGE instead of 3G). So you can buy unlocked phones that are not owned and controlled by your carrier, with no contracts, giving you more power to vote with your wallet if your carrier is unreasonable.
For these reasons, I decided to go with an unlocked GSM phone (Nexus One). AT&T has coverage where I live; T-Mobile does not. Thus, AT&T is pretty much the only reasonable choice.
Are there actually people for whom being on AT&T is a necessity, let alone desirable?