WipePIN protects your PIN from being retrieved using ShowPIN. It runs invisibly in the background. Every three minutes, it wipes the PIN from the registry if it is stored there, replacing it with "????". We have not yet observed any ill effects from doing this. WipePIN runs fine from the system startup folder, and is very lightweight (4 kb).
http://wiki.xda-developers.com/uploads/wipepin.exe