My TF V6 seems to idle slightly high, even with the idle screw turned all the way in. Idle is very smooth, doesn't seem to be a vacuum leak.
When I rev the motor the revs drop back down slowly.

Anybody know what could cause this?

hmmm try disconnecting your battery for 10 minutes then reconnect.that may may reset the cars idle, usually once warmed up it should be btw 600 and 800 rpm

Yeah I have done that a few times, its like a carby motor when its running rich. Maybe the 02 sensor?