Marc, HSPA+ is technically a 3.XG technology, I wish they never called it 4G, because its confusing. It uses the same antenna, chipset, and frequency as UMTS. Essentially what AT&T did was take a Desire HD and replaced the H icon with an H+ icon. The issue here is that (to my knowledge) Android doesn't distinguish between the different flavors of 3G. The icons you will see will be G for GPRS, E for EDGE, H+ for UMTS "regular 3G"/HSPDA. Brandon hit on this in his HSPA+ rant. Despite the Mobile Network State saying UMTS, the H+ icon showed in the taskbar. The same applies for T-Mobile. For example, when you move from an area with HSPA+ to an area with vanilla UMTS (if they still exist) I bet H+ will still be seen in the taskbar.
For phones with separate radios for 4G (ie. WiMax - Sprint, LTE - Verizon) Android is able to distinguish between those because its a different radio, different antenna, riding on a different frequency, as well as being a different connection option...similar to WiFi on these devices.