This one isn't for the faint of heart, but I stumbled upon Rui Carmo's HowTo on making your own iSync Plug-in.

Rui runs the amazing website/blog The Tao of Mac. I can't find the post date, but he's updated his HowTo on making your own iSync phone conduit plug-in.

If you've ever tinkered around with .plist files on your Mac, you might be interested in this writeup. It gives a lot of background on how iSync works with phones, and why some phones just don't work.

If you've got a phone (especially a Motorola, Sony Ericsson, or Nokia) that isn't supported by iSync, yet, you might want to take a look and give it a shot yourself!

If you create an iSync plug-in for an unsupported phone, please email me and I'll post it to the site. This would be a great grassroots way of adding support for manufacturer-unsupported phones. Bottom-up is always better!

Rui also put his hand out for a donation for this useful information. Please let him know how useful you found this information.