AT&T has confirmed they'll be using their newly announced T-Mobile US acquisition -- more specifically T-Mo's 1700MHz AWS spectrum (currently providing 3G and "4G" HSPA+) -- for a bigger, better LTE rollout ("4GS?"). See the slide above for details.
Apple hasn't announced any plans for an LTE iPhone or iPad yet but as the technology matures and coverage improves they almost certainly will. Meanwhile we can hope for better HSPA+ support in iPhone 5, as current iPhone and iPad radios max out at 7.2Mbps.
(See TiPb's guide to wireless networking terms for what all that alphanumeric soup means.)