Just had an excellent resource passed on to me regarding iPod touch and iPad apps. There are literally thousands of apps out there. It would be a full time job just trying to sort through them all. Luckily there is a guy who does this for you. He handles all of the training on iPod touches for his school and posts his resources on his website. The site is http://ipodtouchtraining.weebly.com/.
On a separate note, I had a chance to check out Proloquo2Go on the iPad. My first impression is very positive.