Hi Ralph,
Thanks for the welcome. Sorry about the delay in replying, i didn't notice that you'd replied to my post.
Anyway, further research seems to suggest that it
isn't actually possible to do audio/video between Trillian and iChat via Jabber. At least i haven't found anyone claiming that they have managed it.
It should be trivial to add to Trillian though since apparently iChat does audio/video for AIM, Bonjour and Jabber in exactly the same way, it's just how the streams are negotiated that differs. I suggest that the original poster contact Cerulean Studios (
http://www.ceruleanstudios.com) and request the feature to be added. Alternatively, since Cerulean Studios have pledged support for Jingle, he could contact Apple and request that they support the Jingle standards. ^_^
So it looks like there is currently no way to have an audio/video chat between OS X and Windows users via Jabber. However the future isn't looking so bleak.
OpenWengo (
http://www.openwengo.org) is building a VoIP client that can IM via various protocols, including Jabber, and does both audio and video via SIP. The (soon to be released) 2.0 version will only do audio/video for people using Wengo as their SIP provider, but their FAQ says that allowing you to use the client with other SIP providers is a priority and will be implemented in the next version. So if the OP is willing to set up both Jabber and SIP servers this could be a solution a couple of months down the line.
The 2.0 version of Gizmo (
http://www.gizmoproject.com) already allows you to log into other SIP servers and it uses Jabber for IM, but the Jabber portion is locked to Gizmo (a feature request might yield some results). It currently lacks video support (though i'm sure it's being worked on), but a benefit is that the client supports Phil Zimmerman's Zfone plugin which allows you to encrypt the voice aspect of you communication. Gizmo also doesn't treat Mac users like second-class citizens, our client is generally nicer and has feature parity with the Windows version.
Even if one of these solutions can be made to fit the OP's needs he should still file a feature request with Apple to support Jingle. I firmly believe that XMPP/Jabber will become the standard for IM in the next few years, and it's important that Apple follows the standards so that no matter what OS a person is on they will be able to communicate effortlessly, in whatever form they want, with anyone else on the 'net.
*off to file a feature request with Apple*
iBook G4 Mac OS X (10.4.6)