it should. if you mix and match, it should just clock the faster ram down to the slower speed (ie if you had an 800mhz stick and a 667mhz, they would both run @ 667)
edit: A little further reading indicates that the early 2008 (penryn MBPs) are a bit more finicky when it comes to accepting 800mhz sticks vs the merom SR in the late 2007 revision. Just keep that in mind.