I tried restarting Firefox, and desktop sharing works! I'm using the standard Java 1.6 plugin that comes with Ubuntu 8.10- anyway, glad it works now. We can continue this conversation via email.
Edit: Eh, it ended up making Firefox pretty unresponsive in the end, but I did see a screenshot of my desktop for a while, at least. :-)
The invite link should just work; no other setup is needed. Can you email me the link?