It used to be you could depend on this, and it was great being able to stroll down Main Street before the rest of the park opened. With the drop in attendance since 9/11, though, they don't do this much anymore. Of course, with Disney the answer is never just "yes" or "no." It apparently still happens, but very occasionally.
We went Easter week last year, and MK actually didn't open until a few minutes past the stated time. But one morning at AK we arrived before opening to find them letting people in. They said this was because they expected a very large crowd that day.