I made my own iSync plug-in for the Sony Ericsson K800i, and thought I'd share it [20kb download]. The calendar and phone book both work perfectly well with this plug-in. Extract the tar file into /Library -> PhonePlugins. If you don't have a PhonePlugins folder in /Library folder, then create it first.
Then just pair your device with the computer and then start up iSync. When you add a new device, it should detect your phone and should say it's supported. Hope it works for you too!
[robg adds: I haven't tested this one.]
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