It's pretty rare a NIC allows two letter domains so it might be a general limitation, but they are possible. Either that, or our domain (ep) is just a very persistent hallucination.
I'm tempted but I already own "no.gd" (a rather undiscovered TLD if there were one - and they allow 2 letter domains!) and have little enough use for it already..
.AG .CX .GD .GS .GL .HT .LC .MS .MU .PE .TC .TL .VG
I also heard about .gg and .je (Guernsey and Jersey - British Crown Dependencies off of the French coast) which can be registered directly at http://www.channelisles.net/applic/application.shtml (but they handle them manually so you have to wait a few days).