I had this issue with one of my school's MacBook's, and luckily it was still covered under warranty. I called Apple, described the problem, told them the trouble-shooting steps I took, and they asked me to send them the unit. And I had 10.6.7 running on it. So Apple is looking at it as of today, and hopefully I'll have the computer back next week.
If anyone knows of a solution here, I would love to know what it is.