If you have a Mac, this is a fascinating app that will show your GPS location history based on your iPhone sync backups. There has been a lot of discussion around this app about privacy, what is being recorded without "opting in", etc., but to actually see the raw data is an eye opener.
BTW, this is an open-source project. The app maps the information that your iPhone is recording about your "movements" based on the file in your iPhone backup area. The app does not
record anything in itself, but provides a nice mapping history "video" of your own history.
To try out the application (again, it only runs on MacOS right now), and read the FAQ, visit http://petewarden.github.com/iPhoneTracker/
[Map shown: Cimarron's iPhone 4 all-time history - Boston and NYC seem to be my favorite haunts]