I may be missing the obvious but I thought (and assumed) it was just UMTS (3G). It says here it's the faster (Up to 20Mbit) HSDPA. Just letting you know incase you made the same mistake as me. 
==Quoted from http://www.apple.com/iphone/features/wireless.html ==
iPhone 3G uses a technology protocol called HSDPA (High-Speed Downlink Packet Access) to download data fast over UMTS (Universal Mobile Telecommunications System) networks. Email attachments and web pages load twice as fast on 3G networks as on 2G EDGE networks.1 And since iPhone 3G seamlessly switches between EDGE, faster 3G, and even faster Wi-Fi, you always get the best speeds possible.
iPhone 3G delivers UMTS, HSDPA, GSM, Wi-Fi, EDGE, GPS, and Bluetooth 2.0 + EDR in one compact device using only two antennas. Clever iPhone engineering integrates those antennas into a few unexpected places: the metal ring around the camera, the audio jack, the metal screen bezel, and the iPhone circuitry itself. And intelligent iPhone power management technology gives you up to 5 hours of talk time over 3G networks.2 Thats some of the best in the business.
==Quoted from http://www.apple.com/iphone/features/wireless.html ==
iPhone 3G uses a technology protocol called HSDPA (High-Speed Downlink Packet Access) to download data fast over UMTS (Universal Mobile Telecommunications System) networks. Email attachments and web pages load twice as fast on 3G networks as on 2G EDGE networks.1 And since iPhone 3G seamlessly switches between EDGE, faster 3G, and even faster Wi-Fi, you always get the best speeds possible.
iPhone 3G delivers UMTS, HSDPA, GSM, Wi-Fi, EDGE, GPS, and Bluetooth 2.0 + EDR in one compact device using only two antennas. Clever iPhone engineering integrates those antennas into a few unexpected places: the metal ring around the camera, the audio jack, the metal screen bezel, and the iPhone circuitry itself. And intelligent iPhone power management technology gives you up to 5 hours of talk time over 3G networks.2 Thats some of the best in the business.
