Using applications in the classroom is very challenging, especially with nearly 40 students in some rooms. I suggest you only use one and hook that up to an overhead for sharing. It would be nice to build learning centers around them, but monitoring them would be a huge challenge.
Another item to consider: why use an iPod/iPhone or Android version when a computer is just as easy? The answer is in the various applications and the ability for cell phones to bypass overly restrictive school board web washers. However, be sure you have previewed the material to make sure it is safe to use.
The future is clearly in your hands, pun intended, and the creation of applications is just beginning to show some maturity. For the most part, apps are silly games or material you could easily live without. But in some cases they offer a change to add depth to lessons, make individualizing a lesson easier, and save on the cost of supplementary materials. And, if you find a good one, just e-mail me your find by going to the home page and clicking on my name.
Apple offers some discounts, but these may end without much warning.
Android applications for teachers
Read books on the web -- excellent for teachers without classroom sets or for extra credit or remedial assignments.