[plchan]

I wonder if I purchase an ATT BB 8820 (Wifi and GPS), will it work with TMO UMA or not? Any softwares I need to flash in order to get it working?

I don't think it is hardware related; rather than that, it is more like the OS / software enable UMA feature. Is it correct?

[Helmdawg]

No, UMA is specific to T-mobile.

[John Clark]

Helmdawg is correct. Even if you use the 8820 on TMobile it won't work for UMA. It's carrier specific AND device specific. Currently, the only BB that will work with UMA on TMo is the 8320.