Our German friend from open.source.mac have published an interesting news about iSync plugins, and especially those supposed to be released for some popular Sony Ericsson models. As the company failed to deliver them on time, they decided to use the iSync Plug-In Maker, an application available from Apple Developer Tools, to create a Sony Ericsson K800i plugin, 100% compatible with the one provided by Apple, and demonstrating the power of this tools.
If you own a SE mobile phone not supported in iSync yet, then you should definitely have a look at this story as it also includes links to "home-made" plug-ins and a lot of information (such as HOWTO: Write iSync Phone Plugins)