of course there's going to be some sort of correlation. smart people generally do (relatively) well on standardized tests. hence, a high act score and a comparably high mcat score pair together pretty logically. however, you must take into account the different testing populations: almost everyone who takes the mcat is moderately bright (you know, at least in the 60th or 70th sat/act percentile), whereas w/ the sat/act, it's really a crapshoot.
that being said, the rule of thumb i've heard is act score +/- 3 points... but a 6 point range doesn't provide much in the way of predictive value.