I don't think they correlate all that well. For one, most everyone that gets into medical school does well enough... to get into medical school. For that reason, that's why there are so many more people that pass and do well on Step 1 than don't get a good score on the MCAT. The average Step 1 score keeps increasing, too, which tells you something there.
And now my anecdote. The top student in my class, with a 260+ Step 1, got a 23 on the MCAT.
"Some of it's magic and some of it's tragic but I had a good life all the way."<br />- He Went to Paris by Jimmy Buffett
I definitely believe that there is a correlation between MCAT scores and USMLE scores. However, the correlation isn't particularly strong for a number of reasons. The tests both assess knowledge of topics, but there is far less reasoning on the USMLE. In my experience, questions on the USMLE are very straight forward and only require recognizing a pattern. The amount of knowledge required is immense, but you do not need to apply the knowledge in the same way the MCAT requires. In some ways, the MCAT is probably better at predicting the clinical skill of a physician than the USMLE which is gauging how much information you are able to absorb. I did above average on both but I found the MCAT to be more interesting and stimulating than the USMLE. Test taking skills obviously benefit you on both exams as well which can explain part of the correlation.