Have you tried the game 'Memory loss' developed by dinoraur? I was quite skeptic about purchasing it at first, but the game is really good and it's fun to play too... Sharpens up your memory, easy to play! The difficulty level increases as you unlock the next levels of the game. The higher you go the more interesting it gets. It is out in the apple app store now; that is where I purchased it from and I have heard it will be available on symbian and android by dec 2010. Really addictive game! Do take a look at it! You can view the gameplay on youtube as well.
Memory Loss for iPhone, iPod touch, and iPad on the iTunes App Store