Rui Carmo...for which I'm sure to get karmic compensation soon, probably under the shape of a very early morning conference call or something similar.
Anyway, I've just posted a pretty drastic overhaul of my HOWTO on writing iSync phone plugins, wherein I fix most of the things I had figured out wrong.
I have since created another few test plugins and figured out more about how iSync works, although I haven't yet fixed the one I posted for the Nokia 9300 (Dave Bourke was very kind to send me a bunch of logs that I have been going through, and I will be updating the file soon).
In a nutshell, my earlier intuition was right: Info.plist must have a set of unique IDs for the plugin bundle and the IPHPhoneConduitVersion key set to 400.0 (or above) for iSync 2.2.
The rest seems to be business as usual: picking the right phone family to inherit from, tweaking the serial and USB IDs, etc.
What did hit me completely by surprise was the inclusion of what appears to be JavaScript inside MetaClasses.plist. Apple, could we have some official documentation now, please?