Took a lot of reading and hacking around but finally got it down pat. Here is how you can use mp3's as ringtones on your m610 without connecting to the internet or paying for any proprietary music service:
1) Select an mp3 from the music files on your Mac machine. Open it up in itunes.
2) Select the file in itunes > right click on the filename and choose 'Convert Selection to AAC'
3) Locate the new file, probably saved to your itunes library under 'Unknown Album>unknown artist
4) Open this file in an MP3 editor program - a good one that you can get for free is Audion - http://www.panic.com/audion/
I used protools.
5) Use Audion (or mp3 editor of your choice) to cut the mp3 down to 30 seconds or less, also make sure the file ends up less than 500kb or it won't be usable.
6) Rename this new file something that makes sense to you, but make sure it has the extension .3g2 - instead of mp3/mp4. This is essential.
7) Open 'Blue Tooth File Exchange' on your Mac > choose 'send file' - select the file then choose your cellphone from the list of discoverable devices (if you haven't already paired your phone to your mac via bluetooth search the forums for how to do this)
7.5) You must set your exchange ftp folder to dcim and its subfolder and place the file right next to your other jpg's or it won't work.
8)Cool Once your new 3g2 files has finished uploading to your cellphone, go into 'settings>sounds>Ringer Type>Voice Calls>with caller id and when prompted choose 'my videos' (trust me, it doesn't make any sense because it's not a video, but it works Smile
9) Your newly uploaded 3g2 file should be represented by a white square with an 'X' in it > set this as the ringtone and as long as you've done everything right up to this point you should now have an MP3 ringtone!