While we're still hearing conflicting rumors about super-fast LTE 4G networking for iPad 3 this spring or iPhone 5 this summer/fall, you'll be happy to hear that AT&T is preparing to replace their current microSIM card stock with LTE compatible versions.
Now the Andorid-powered Pantech Burst and the Windows Phone Nokia Lumia 900 are , which is likely to be landing on AT&T really soon. An internal memo has instructed sales repts to scrap all of the old microSIM cards, and use these new LTE ones with the same SKU.
Okay, so maybe the Pantech Burst and Nokia Lumia 900 seem a bit more likely, but there has been an awful lot of solid-ish talk about the iPad 3 lately, and even if we don't see an LTE iOS tablet soon, WWDC is looming along with the possibility of an iPhone 5. Whatever comes out next, it's hard to imagine it not having at least one model with LTE connectivity, and even less likely that they'll ditch the micro SIMformat.
Verizon switched to LTE microSIM cards in October for the Samsung Stratosphere, so they'd be all set to go with any new Apple hardware as well. Sprint is still getting its feet wet with LTE through the Galaxy Nexus, so they may be next in line to start offering LTE microSIM cards -- we'll be keeping an eye out.
If there's no LTE in your area, we can understand if this doesn't matter very much to you... yet. But when it does come, and you start pulling 20-50mbps of data, you'll probably start to care a lot more.