We've all heard about the how your iPhone tracks your location and keeps an unencrypted file of everywhere you have been. If you upgrade to IOS 3.3 this will be fixed. I'm not going to go into the debates about this 'feature/bug' but before you upgrade, why don't you just download the iPhoneTracker application and have a look for yourself. It is amazing.
I tried the Mac OS X version from here: http://petewarden.github.com/iPhoneTracker/ but you can also download the Windows version from the same site.
Be prepared for surprises and whatever you do, don't look at your husband's or your wife's or you kid's iPhone ;-)