Getting access to your iPhone's file system can be pretty difficult. But if you're a Windows iPhone user, you can take advantage of the free BSD-licensed IPhone file system browser iPhoneBrowser. You can browse your IPhone's files, drag and drop stuff onto it, and copy your MP3s and movies back to your hard drive. iPhoneBrowser also offers a backup system for keeping your iPhone files safe.
It even works with non-jailbroken iPhones, but as the developer says, you won't get very far with one, thanks to Apple's file system "sandbox".