The unlocked Nokia N900 retails for $649 + tax, with a free BH-703 Nokia Bluetooth headset, and shipping early October. Note however that while the N900 will work on both T-Mobile and AT&T for voice calls, 3G data speed will only work on T-mobile. The slower EDGE data speed might work with AT&T though.
There is no word yet if T-mobile will officially carry the Nokia N900. If they do, you might want to wait a while as they will surely offer it at a lower, subsidized price.