Martin Sauter has a nice post titled HSPA About to Overtake Wi-Fi 802.11g. He points out 802.11g, with a theoretical data rate of 54 Mega bits per second (Mbps) but a practical throughput of only about 20 Mbps, will soon be obsolete when compared to coming to HSPA+ data rates of between 21 and 28 M bps.
If you are not familiar - HSPA+ is also referred to as Evolved High-Speed Packet Access and is basically an enhanced 3G wireless broadband standard formally known as 3GPP release 7. It's just starting to take off - according to GSM World, there are currently 20 HSPA+ networks running at 21 Mbps and two running at 28 Mbps in the world today.
I think I'm already finding myself making the transition on my iPhone 3GS. When I'm away from my home I usually don't have the WiFi radio turned on, running off the 3G network connection. I do this to save battery and also have some concerns about security on open WiFi networks that I may end up attaching to. I usually don't notice a difference in performance and sometimes find myself checking the top of my screen to see if WiFi is turned on by mistake. What kind of bandwidth am I getting? Here's an iPhone screen shot that seems pretty typical - 1.265 Mbps downstream and 348 Kbps upstream. Not bad for a device I carry around in my shirt pocket (as a comparison a T1 connection runs at 1.544 Mbps) and.... it's going to get a lot better.