Looks like T-Mobile is still out. From the iPhone 4 spec page : UMTS/HSDPA/HSUPA (850, 900, 1900, 2100 MHz)

Heres the common 3G bands and where they are used:
Originally Posted By: Wikipedia

Band I (W-CDMA 2100) in Europe, Africa, Asia, Australia (all carriers' metropolitan networks), New Zealand (ITU Region 1) and Brazil (part of ITU Region 2)
Band II (W-CDMA 1900) in North America and South America (ITU Region 2)
Band IV (W-CDMA 1700 or Advanced Wireless Services) in the United States (T-Mobile USA) and Canada (WIND Mobile, Mobilicity)
Band V (W-CDMA 850) in Australia (Telstra NextG Network), New Zealand (XT Mobile Network), Brazil, Canada, the USA, Guatemala, Costa Rica, Venezuela, other parts of South America, Israel[2], parts of Asia (ITU Region 2 and ITU Region 3), Poland
Band VIII (W-CDMA 900) in Europe, Asia, Australia (Optus and Vodafone regional/country 3G networks), New Zealand (ITU Region 1 and ITU Region 3), and Venezuela (Corporación Digitel, C.A.)


Maybe iPhone 5 will go with penta-band.