The new program to beat on the block is Moi3d.com. If you haven't seen this NURBS CAD modeller, it is really being praised high. It's designed from ground-up to work with tablets. ZBrush, Sketch-up, Blender (sculpt only), etc - users want to just use a tablet.
Note that making 2+ buttons _useful_ is different from making them mandatory.
A one-button/stylus will work with touch-senstive screens too. Think PocketPC or UMPC (or even TabletPC), iPhone/Touch, Palm, Nokia Nxxx series, etc. The size of touch screens is growing - and notebooks and touch screens are both growing fast in popularity. A touch-screen CAD program could see a lot of mobile use. I'd love to run it on my N800 :)
By the way, I haven't had a chance to try AvoCADo out yet because I can only use Java 1.5... and will for the forseeable future. Many corporate or non-computer-tech people can't run your program due to Java 6. I don't know what features you actually need, but you should reconsider if whether it's worth losing a lot of potential users.