I agree that the A.I seems to have not been improved since Prologue, but I'm really hoping the builds shown so far are holding back as suggested by some things KY has said, including what he has said about the GT5 TT.
I find myself racing in GT5P against the A.I more than humans online because of connection errors, long matching, and usuallly I'm placed in a race with either 1 other driver who spins out every lap who I wait for constantly, or a pack of wreckless drivers who wreck me on the first lap.
My problems with the A.I in GT5P are less than my problems with A.I in GT4, but the A.I still seems to always force me off the driving line or track if I'm beside them, they also brake later than me yet make the turn (with the same car) as well as start accelerating earlier, while taking the exact same lines.. to the point that even a car that loses control in the race will do it on the same corner every lap.
I really hope for more dynamic A.I, and before I finish my post I'd also like to point out another problem I have with the A.I is on the Daytona Road Course on the long straights as I am faster no matter what car I'm driving which makes it seem like the A.I is going 90-95% throttle on these sections making long races on this track boring as I will lap my opponents.