I thought about this this past week when I had to go to the local uni's CS department for something. I peeked into a half dozen classrooms, lots of laptops, mostly dells, a handful of Toshibas, less than 10 MacBooks...the most I saw in one class was 3.
Likewise when I went to the student union to grab some dinner. Hundreds of students all with laptops open, a handful of Macs. I'd say at rates about at the same as market penetration...~10%.
It may be a geographic thing. Certain regions may make MBPs more appealing, or certain schools may subsidize the purchase more. I've noticed similar rates at other schools in the area.