It would good to create a collection of these if people are aware of them. Send them through and I will add them to this article. These could even be cited when making the argument for implementing such programs.
Here are some of references to existing research that I found interesting.
This site has a collection of links to what other research institutions are also doing:
This issue will become more of a requirement as legislators start to ask educator to justify continued financial support of such programs and as parents and the community ask to see evidence of the benefits of these changes to their children's schooling. Some people have started this process.
As a start we should be evaluating the apps we chose to use with our students. Kathy Schrock's has started to put together a couple of scaffolds for doing just this. Her site, always a source of fantastic resources has a great section on iPads.
Another fantastic is resource is Apps+Taskonomy. The guys at Balancedtech have put together this PD Session that could be completed by anybody in their own school. They have hyperlinked all the necessary resources into the PD material. They also have a plethora of iPad resources on their site.