Well, believe it or not, some movies out there are not synced very well, and I think I've read about certain DVDs being very bad. Yes, there is a standard for sync/time, but obviously they don't all adhere very well.
However, regarding Auto Motion Plus, that is an algorithm for Frame Interpolation. It will try to take your 24fps source, or 60fps source, and then try to fake the inbetween frames. I suppose, say your tv is natively 120fps. That means with 24 (read: bluray and dvd) there is a the potential possibility of faking out 96 frames for every second. Can you imagine how much processing power that would take? Then again, if it's being done per true second, I guess there still shouldn't be any sync issues.
You need to calibrate for your sync in your receiver, and for every* source too.
Try to get your hands on a DVE bluray, I believe it has something for sync.
Otherwise, you gotta eyeball it to some reference. Google "clapperboard lip sync".
The last straw is to not pay attention to it. I was at my brother's recently, and the sync was out of whack. Not insanely bad, but enough for me to notice. I just decided not to notice anymore.