Had the same problem on my 08 3.0 si. Reported it to the dealer and had it booked in for that and other issues.

The delaer reported no fault (surprise, surprise) and just reset the learning module.

I must say though, since then I have not experienced the lag!

My theory is that when your car is new most people tend to 'baby' their car and are very gentle on the throttle (running in the engine). Hence the car thinks its your granny driving it. Since the reset I have not had to pay attention to the running in process and give the gas a bit of welly. Therefore the transmission is now learning my usual driving habits.

Does this make any sense? But since the reset, have noticed huge improvement.