An MP3 ringtone will work if you're using iToner (Mac). If you're on a PC platform, I hear that iPhoneRingToneMaker has been updated to work with the current firmware. That software will also let you edit your MP3 before you make a ringtone from it.
Personally, I use iToner and have non-musical MP3 ringtones (e.g., CTU, Uniphone, etc).
--DotComCTO