Why pay full price for a Nexsus One designed for ATT when you can get a New iPhone in June/July subsidized at $299? You'll be with ATT anyway for 2 years since T mobile 3G is piss/poor.
You must REALLY love the nexsus one.
To listen to many, AT&T is piss poor in much of the country as well. Others will tell you that T-Mobile is fantastic. Cell phones service and location are both fully dependent on "location, location, location" for perception of value and quality. It really just depends on where you use the phone.
I don't know what AT&T's contract-free prices are, or will be, if you come to them with an unlocked Nexus One, but on T-Mobile, buying upfront and contract-free saves you $20/mo on the monthly rate for the exact same service plans as the contracted plans, so you in effect save $130 and break even on the phone about 6 months earlier than the contracted folks.