Did a search and didn't see anyone mention this app so I'm guessing it might be sort of new. But I've been looking for something like this for a while (as I'm sure many of you all have too). This app allows you to set a password for any app on your phone. It also has a feature to disable all the passwords so you don't have to type it in every single time if noone will be going through your phone or whatever. There's a free 7 day trial but after that it's only $.99. I just went ahead and bought it because I've been wanting this forever haha.
Saw this on Droid-Life, went ahead and bought it. Nice app, perfect for keeping my little kids out of critical apps like Rom Manager and Root Explorer.