Tiger iChat has Jabber support
James pointed me towards this post on AppleInsider when I asked about multi-network support in iChat:
In addition to supporting video conferencing with up to three people, the most recent builds of iChat 3.0 include support for Jabber's real-time communications software. The iChat implementation will let organizations host their own Jabber servers, allowing employees to use iChat privately and securely behind a local firewall. A new addition to the iChat setup assistant provides an interface for enabling Jabber support. AppleInsider: New Mac OS X Tiger builds reveal iChat Jabber support, parental controls
So, it looks like iChat will support Jabber. Which means, of course, that if you or your organization run a Jabber server with the proper gateway installed, you can appear online using several different networks -- MSN, Yahoo, etc. Not nearly as good as the single client support for all networks found in apps like Adium, but a step in the right direction.