Apple won't be shipping the 27" iMac until year-end and Tim Cook has stated that supplies will be constrained even then.
So sounds like the rumors of Apple having yield problems with mating the 27" LCD with the iMac case are true and this problem would affect the ATD, as well.
So I don't see a new ATD until the 27" iMac production is in full swing and they can fill the channel, allowing them to divert panels to ATD production.
A launch in conjunction with a TB-equipped Mac Pro, as sdo1982 suggested, seems logical to me. With the iPad 4 just launched, Apple needs something to talk about in March. 🙂