Today on b3ta I came across a flash version of Tower Defense that I've just spent the last 45 minutes playing (I got to level 27. Can anyone out there top that?).
If anyone from the mobile or casual (*ahem* Kim *ahem*) space is reading this, if you haven't already done so, take a look at Tower Defense as an excellent example of 1-button gameplay done right. The core concept is very simple and easy to grasp, the controls and be dumbed down to the most basic of input devices, but there is plenty of depth available. Throw in multiplayer and controlable heroes (as in the WC3 mod) and you have something that can keep the attention of casual and hardcore players for hours.
Update The always excellent Jay Is Games did a review of this title a couple of days ago and the forums show some good strategies. My ego always falls about 5 notches after reading the forums on JIG as these people always have stratospheric scores.