If the Jabber server were integrated with OpenACS, you could just let
people use their regular IM account (provided when they register) and
Jabber would be responsible for telling OpenACS who's online. That
way you could get cute little icons next to someone's name when
they're online.
And then for people who haven't been introduced to the wonder of IM,
you could have a link to the Java Jabber pop-up window. However, I'd
probably just tell them to go get an AIM account instead of dealing
with signing java applets. You can always use AOL's java client if you
aren't at your own terminal.