I can't find anything that indicates the '06 WRX has anything other than the same TD04-13G that the '02-'05 WRX came with. But I can't guarentee it...
You thinking about update/backdate? Is the '06 gonna be on the same line as the '02? The move to the 2.5L motor is pretty significant... I'd probably wait a bit to make sure no one challenges the rule. It would suck to go to the 2.5L WRX motor only to find out you're now in SM, if your goal is to stay in ESP/STX... (actually, doesn't the 2.5L WRX have to go to STU now?)
Also, the update to the '06 is gonna be pretty expensive... IIRC the rules require you to update the motor as a whole unit, no? That means you're gonna need the long-block (since the 2.5L WRX has different AVCS heads), the intake manifold (convert to throttle-by-wire), and a new engine wiring harness/ECU (to control the AVCS and throttle-by-wire). Hell, the swap I'm doing will probably cost about the same, and I'm getting the 6-speed!
Either way, I guarentee someone will do it... and someone will protest... I wonder where this will end up in the rules when all the dust settles!