4G LTE is available 3 phones for ATT: LG Nitro HD, Samsung Galaxy S II Skyrocket, and HTC Vivid. ATT has been releasing LTE services in major cities and their surroundings and will continue to (they estimate that, by the the end of 2011, 70 mill. of the 300 mill. of Americans will have LTE capability).
There is sort of an increase in 4G speeds to 3G. While it is definitely an upgrade to 3G, sometimes it is hard to spot the difference in speeds, sometimes it is quite obvious. I think it would be safe to say to buy an LTE phone and reap the benefits of the ongoing deployment. Worse case scenario, you live where 4G LTE isn't available and still use 4G. Worser case, 4G isn't available, and you still have a fast (maybe that's pushing it, but) browser experience with 3G. Worst case, you live in the middle of nowhere and get freaking Edge on your phone, 2G, which goes at a woefully slow pace.
ATT FTW!