Macworld reader Nick Dunklee points out that a teardown at RapidRepair of an iPhone 3GS shows that it has a UMTS/HSDPA chip. UMTS is the earliest 3G standard deployed on GSM networks, and it tops out at 384 Kbps. Its easy to test, if you have an iPhone 3GS. Go to any speed tester, like Testmyiphone when youre outdoors with a good signal. Downstream, you might hit well over 1 Mbps; upstream, under 384 Kbps.
Unfortunately this seriously limits the upload speed of your iPhone. Those who upload photos and video regularly will not be able to take advantage of the higher speed network.
Its likely that Apple will introduce this in a future model of the iPhone as a 'New Feature'.