Now, it looks like AT&T will be officially launching the Galaxy Nexus, complete with LTE support. This is still an unconfirmed rumor at this point, but AndroidRevolution is claiming that an "inside source" at AT&T has said that AT&T SIM cards for the Galaxy Nexus have been sent out for testing, which is usually a pretty good indicator that a device will be coming relatively soon. The SIM cards have been confirmed as LTE-enabled. It has been reported that the SIM cards are full-sized and not micro-SIMs.
We can't say for sure how reliable the source is, but if all the info checks out, it seems likely that an LTE-enabled Galaxy Nexus would be making its way to AT&T fairly soon. We'll try getting some more confirmation and bring you more as we get it.