The only Nokia that will work with Cingular's 3G is the N75, which is not currently out.
There are 3 frequency combinations for 3G:
Europe: UMTS 2100 (uplink and down)
Cingular UMTS 850/1900 (uplink and down, but not dedicated. I.e. you could use UMTS 850 for both up and down, or 1900 for both up and down)
T-mobile US: UMTS 2100 (but with 1700 uplink, therefore incompatible with European 3G phones)
The FCC is to blame for the confusion.
As to why your N73 showed the 3G bar, I believe that if you will go into Settings, Network Settings, You can select Dual Mode, UMTS Only, or GSM Only. You probably have Dual Mode, which means that the phone will prefer to search for a 3G signal, and use GSM as a backup. Thus when it has no signal, it thinks it's searching for a 3G signal (it won't find a usable one). You should set it to GSM only in the States, to prevent your device from unnecessarily searching for a 3G signal.
The N75 is the only announced US 3G phone. There are rumors that there will be US versions of both the N95 and E90, but as of yet they are merely unsubstantiated rumors.