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.
A free Mac application called iFuntastic (via MyiTablet) also claims to provide a step by step interface but early reports appear some mixed results. The requirements are an Intel Mac.