On this topic, I've come to several conclusions.
1. There is a weak correlation between SAT & MCAT scores.
2. The tests are very different in their approaches (e.g., SAT Verbal = heavy emphasis on vocabulary, MCAT Verbal = heavy emphasis on reading comprehension).
3. A particular person's circumstances (e.g., how motivated s/he was to study at the time) trump any predictive power of the correlation.
In English, if you scored well on the SAT, you stand a fair shot at scoring well on the MCAT. Or, if you did poorly on the SAT, you can still do very well on the MCAT.