So, as a test, I chatted with myself using two Jabber IDs and servers, one logged in via Adium and the other iChat. It worked perfectly. This tells me that it's not the ports nor firewalls, it's not iChat, it's not anything I can do on my iMac. It makes me think that the issue is the Jabber servers. <sigh>
The reason I sigh is that I've subscribed at five Jabber servers, and none work so far for me. There were the two Melvin suggested (freenet.de and netlabs), jabber.org.uk, sweeney.ath.cx, and swissjabber.org. I've had problems with not all of them working for both protocols I want to use (MSN and Yahoo) as well as this mostly one-way issue.
So does anyone have any suggestions? Signing up for every Jabber server out there until I can get one to work with iChat doesn't seem too appealing.