I recommend you a nice tool to deal with your ipnone ringtone. It can convert MP3 to iPhone Ringtone, even convert any video/audio to iPhone M4R Ringtone, including 3GP, AVI, MKV, RM, MOV, XviD,MPEG, WMV, DivX, MP4, H.264/AVC, AVCHD, WMA, WAV, RA, M4A, MP3 to M4R iPhone ringtone. Most pleasantly, you can put the converted M4R Ringtone on the iPhone directly without iTunes.

4Videosoft iPhone Ringtone Maker