Or you could buy the regular ones but tell the students they can be tracked anyway :-)
You can also track with wifi as Apple have a database of all wifi APs around the country as well.
Theft is certainly a talking point which affects any portable device, including laptops and cameras. It's bound to be more of an issue where you have something extra desirable like an iPad or iPod touch. Schools are talking about engraving the metal backs to act as a deterrent to the eBay crowd. If you book them out to students by serial number (printed on the back of each iPad) at least you'll know who the culprits are when some of them don't come back :)
On a positive note we're finally seeing some strategic thinking about this type of device and what they are actually FOR in an educational context. Lots of interesting comment around the fact that students tend to see the activities on them as games rather than lessons, purely because of the type of experience they're getting. Have a look at Apple - Education - Mac, iPod, iPad, and iPhone for learning to see what Apple themselves are saying.