The specific issue that stops the M600, P990 and W950 working with iSync is this:
iSync (over Bluetooth) relies on there being something called "OBEX SyncML Client" and also (the old way) "OBEX IrMC Sync Server" in the phone. Symbian phones do not have this functionality.
Sony Ericssons latest Symbian phones have something called m-router connectivity which is not supported by iSync currently.
For this issue to be resolved in iSync, Apple will first have to add m-router connectivity to iSync. Then third-party manufacturers can add the Phone Plugins to iSync for individual phone models.
There's nothing to stop Sony Ericsson or a clever third-party developer writing a separate application with m-router connectivity built-in. But as iSync is an integral part of the Mac OS, it makes sense to incorporate it into that.
With iSync 2.3 Apple have given handset manufacturers the means to create iSync Phone Plugins for their phones. So, in theory, Sony Ericsson could include a Phone Plugin for iSync in the box with the phone at purchase, and I know that Apple and SE are working towards this.
For Symbian phones, however, Apple first has to add the m-router support to iSync. Who knows how far along, or even if they are working on this?
With Mac OS X 10.4.8 just being released, and 10.5 only a few months away, I wouldn't be surprised if this is a Leopard feature.