The only real difference I can see between our plugins is that you explicitly name the device SyncML Databases, but I use "No target database names in Server Alert" (whatever that means!) It doesn't seem to make any operation difference though.

I switched on debug in iSync (launch it whilst holding down Shift and Option). It generates reams of data in a log file (2Mb per sync), but searching it for any details of the N900 created event turned up nothing. It may be a problem on the N900 where it isn't exposing the event over SyncML for whatever reason, rather than a fault of the plugin or iSync.