I have been hunting for solid tower defense games for my BlackBerry PlayBook. The latest I stumbled across is Jungle Defense by Insane hero. It has some awesome gameplay and is truly fun. The one complaint I have about the game is that it does not take up the whole screen on the PlayBook since it was obviously designed originally for another platform.
Still for $0.99 in App World, Jungle Defense is a great game. Hopefully they will see sales take off and format the game properly for a widescreen device.