There are working corss platform solutions.
Skype works today, but we have several up and coming replacements.
Actualy, your only biggish problems with linux sip clients are coming from varios firwalled/NATed network setups. STUN doesn't work in all scenarios. On linux I've been using KPhone, Ekiga(gnomemeeting), sip communicator (a java client) and some homebrew for a while without major problems. I mean using audio/video conferencing including group conferences. Asterisk and SER are doing a fine job indeed. OpenMCU & friends are a working platform. The real problems are not really technological. They are UI on the client side and lack of sufficient sysadmin skills for deploying decent VOIP setups. The third problem is that more often than not, users on most platforms will need holding their hands a bit until they get the best from their hardware - microphone setups, echo cancellation, etc...
I believe some folks at CivicSpace have occasional access (?) to an Asterisk box. I think we can probably fund an Asterisk box for testing out of Germany, I bet we can get one put in at OSL, and we can probably get another in at a San Fran. I can discuss the same in Cardiff, so we can possibly have a box here as well.
For practical conferencing meetings a good place to research is VRVS, as in vrvs.org - this is Internet2 and High Energy Physicsonline community, which is open to the public. I think it could be possible to have a drupal section there. It is a web portal to H.323, SIP/SDP and VNC. It supports virually any sip or h.323 setup including it used to have sip communicator launchable from their website. Cheers, Vlado