In case you haven't checked this out, Veritas Prep has developed a free GMAT study on-the-go application for the iPhone and iPod Touch. http://www.veritasprep.com/iphone-gmat
There are almost 200 questions that you can practice with on this app, and each one has a detailed solution! On top of that, you can get your performance stats after doing a set of questions to nail down where you're strong and weak on the GMAT.
I've got it on my iPhone and it's definitely a lot more productive than playing Fuzzle or Sudoku when I only have my iPhone to keep me busy while waiting at the airport for my flight or riding on the subway.
If anyone else has used it also, definitely let us know what you guys think and how you think it can be improved!