Following the original 23 Step Ringtone installation tutorial, are a couple of applications which claim to provide a friendly interface to accomplish the same task.
Please remember that all these iPhone software modifications have some risk of turning your iPhone into a $600 paperweight, so proceed at your own risk -- but early reports have shown success.
For Windows users, Efksoft has released an application called iPhoneRingToneMaker which is a $9.95 application that allows you to create ringtones from MP3 files and transfer them to your iPhone. It requires Windows 2000, XP or Vista. A trial version is available, and according to some reports it seems to work.