There has been a lot of debate as of late regarding student use of electronic devices in classes.
Many schools have policies prohibiting the use of mobile phones in class, some don't even allow them in the school at all. Besides issues with students having cameras (or video cameras) with them at all times, there are legitimate concerns about test validity and security as well as time spent off-task in class.
Time off-task in class is also affected by iPod (and other mp3 player) use. In particular, the games and applications that are available on the iPod touch (and presumably the upcoming Zune HD) can be very distracting for students.
On the other hand, there are a number of potential uses for these little computer-like devices, especially if you don't have a computer for each student in your classroom. It will be interesting to see how this all develops.