it all depends on the phone itself, some new phones can import any MP3 file and play it ( what the companies call 'true tone' or something similer)

for the 'regular' ringtones, a cut down midi format is being used, basicly what it means is that u need to choose a pre-decided tempo (from a list) , you need to use GM sounds and you need to save it as a single midi channel (type 0 midi file)/s

you can edit with any midi editor/sequencer, set the instrument to your computer's GM kit, and export as midi type 0.
transfering to the phone depends on the model, it culd be directly thru bluetooth, cable or infrared, or via the net (upload to any server and set your WAP browser to the file's URL)

the format is made by yamaha, since the provide the MIDI sound module for most manufacturers , and it is called SMAF
all the specifications are on their site, including free decoders and loads of info:
http://smaf-yamaha.com/

if u know anything bout midi, just reading the specifications will give u all you need, tho IMO this format will dissapear since new phones can use mp3's like i said...., and support direct connection with pc's, so i assume many user wil start loading (illegal) mp3's instead of DL'ing low grade midi files that dont really sound like the original...

this format BTW is the same format used for mobile games