As a professor (of physics... but often assigned the intro calc-based physics classes for engineers), I will say I've never seen a student with a tablet PC in my classes, though I've seen many laptops.
As a graduate student, we had ONE (out of forty+) in our incoming class have a tablet PC... and the rest of us always felt he was using it less for usefulness and more for "showing off" (since they were quite pricey at that time... and most, if not all, of us didn't even use laptops). He wasn't very popular with our class as a result... and I think that hurt him academically, since a lot of times we would work and study together.
I agree with MATLABdude... even with more and more students owning laptops, most people still take regular notes and do their assignments by hand on paper. (Note that after having a dismal rate with homework submission via an online system, I'm reverting back to paper submission of all my homework assignments.) I think students like laptops because they can take them to the library, coffee-shops, etc... and they also take up less room on a desk (of course maybe my office just needs cleaned so I have better desk space!).