I have been successfully using an iPad 3 with iOS7 Beta 3 for a couple of weeks. I was able to enroll and download apps from Self Service. Aside from a couple of icon problems, everything worked well.

I just reset the iPad and installed Beta 4. I have enrolled the iPad in Casper but cannot download any apps from Self Service.

Just a bit surprised that's all. Will try again when the next Beta is out.