The short answer is: I have absolutely no idea about what's going on right now, and it doesn't look like I will anytime soon due to sheer lack of time.
Michael Simmons pointed me to item #3 on this KB article, which I recommend reading. Item #3 may account for the behaviour you're experiencing (italics are mine) -
3. If you have modified iSync or installed third-party software to support your specific model of phone, or if if iSync 2.2 specifically adds support for your specific model of phone, you should consider removing your phone from iSync, undoing these changes and uninstalling that software before upgrading. Your phone will not work with iSync after applying the Mac OS X 10.4.6 Software Update and your third party software will need to be updated.
I've only updated my iBook so far - and at nearly 2AM, which wasn't exactly the right time for doing extensive testing, although I did spend a bit trying to get the new IPSec L2TP options working (alas, the Cisco VPN client seems to botch exported certificates, and I need those for authentication against my corporate VPN).
Once I have updated my iMac (which is where all phones are synced) and have had time to re-test everything, I'll have a better idea. A friend of mine has also confirmed that old plugins no longer loaded on his PowerBook - we have yet to figure out why (it could be a million things), but I'll have some free time in a few weeks' I can devote to this.
Until then, the HOWTO will have a link to this post.
I did notice, however, that the supported devices page was significantly updated since my last post (I can probably blame that on Netcabo's "transparent" caching proxies, which regularly force me to repeatedly flush the cache and hit Cmd-R to get Slashdot to display correctly), and that most of the devices in my desk drawer (including the N70, the V600i and many others) are now supported - which mostly absolves Apple from my previous rants about their not paying enough attention to the European mobile market.
Anyway, getting back to the initial topic: If you're using the 9300 plugin successfully with iSync 2.2, please drop me a line by e-mail (I've since returned the phone, so I can't test it myself even if I had time to).