http://www.onetouchpass.com/otp_features.html

Seems pretty nice. You use a picture (one of your own or the defaults) and define "hotspots" that you have to press (up to 4 I think) to unlock your PDA.

I believe the new version also has a setting such that it will lock if your PDA is off for a certain amount of time.

Definetly worth playing around with if your concerned about protecting your PDA.