It appears AT&T owns the bands in a few select markets including Phoenix, Las Vegas, Raleigh, Chicago, Dallas, Los Angeles and more and uses it to fill coverage in spots where needed. The coverage area for their share of the Band 4 spectrum is so small that the amount of Nexus 4 owners that might be able to access the LTE network would likely come out to 1% or less.
Its not the wide-spread coverage wed want, of course, but there it is. Its likely AT&T wont gain much more of that spectrum in the near future as T-Mobile is said to be eyeing it for deployment of its own LTE services. If that were to happen then the Nexus 4 certainly could be up and running on a major LTE network in some official capacity by 2013′s end.