I don't really see any carrier ready for 4G. And I don't really saw 4G phones one year ago. If you are fooled by calling HSPA+ 4G, than that's your thing. Technically, this is 3G, or 3.5G if you like. But as the iPad 2 didn't ship with HSPA+, chances are little for the iPhone 5.
At CES, AT&T said it plans to begin its LTE rollout in mid-2011 and have nearly its entire network upgraded to LTE by the end of 2013 (the same time period Verizon plans to have its LTE rollout finished).
Expect it for the iPhone 6, given the chipsets improve. Even though LTE comes in 326MBit/s and 172MBit/s flavor (and with a peak download rate of 100MBit/s), and this is faster than your VDSL or cable line, this is still not 4G. ITU stated 1GBit/s peak(!) downlink as a requirement for 4G. (As no carrier achieve this, they just rebrand whatever they like to 4G).
However, AT&T is pretty much finished rolling HSPA+ out, Apple might consider that.
While surfing, you don't really feel the difference between HSDPA and HSPA+ anyways, as the latency is higher.
On the other hand you can reach your 1GB data cap in 80 seconds with the 100MBit/s peak download rate of LTE. After that, you're with good ol' EDGE anyways. For me, it's first the real 3G flat rate that matters. Once any carrier in the world can do this, it should think about increasing the speed. Anything else is BS. If the carriers wouldn't have in-house DSL/FTTH/DOCSIS, this would be first priority. I hope there's going to be LTE soon though, so VDSL goes away for good and carriers don't hesitate to offer real flat rates. All that blocks progressions there are the $$$ that come in from traditional phone lines, I doubt that there are any technical reasons at all.
Edit:
Vodafone Germany give a good example of LTE. We can expect our cap to raise to 30GB. With their "up to 50MBit/s" statement, that gives us a whooping 1.5 hours of full speed downloading. That's an improvement, huh?
I hope
this model will make it outside of Kansas City and eliminates the need for inner city mobile internet all together with WiFi+VoIP.