I was just asked the question the other day by a teacher, "Is there a way to lock the iPad so that a student can only have access to one app?". After I finished scratching my head and realized I had no idea, I did what I usally do in this situation and run...not walk....to my dear friend Mr. Google. And of course there was my answer! I found not only a blog article by Josh Smith on Gotta Be Mobile but also a You Tube video that he created as well. Hope this will help you out when you need to lock down your iPad to only one app.
*This will not work on the iPad 1, only the 2 or 3.