Regularly syncing an iPhone with iTunes has benefits that exceed keeping your music library up to date. For example, iTunes backs up "Notes" you write on the iPhone 3Gs and iPhone 4 when it backs up your contacts. As a result, if you use one of these devices and accidentally delete an important note, getting it back is as simple as performing a sync.
Connect the iPhone to your computer via its USB cable. Open iTunes if the program doesn't launch by itself.
Double-click the name of the iPhone in iTunes' left menu. Click the "Sync" button located under the "Summary" tab.
Wait until iTunes tells you the sync process is complete. Disconnect the iPhone, then open the "Notes" app and read the note you thought you'd lost for good.