only Gaim is mentioned on this site, but there's at least one more: Miranda (www.miranda-im.org).
the standard plugin set that comes with it makes a perfectly portable app (which is sometimes a pity, like after you move to a lower res video, as it stores window positions in its db).
I'm going to play with Miranda more when I get time. But I haven't even been able to get it working in a normal local install.
Sometimes, the impossible can become possible, if you're awesome!
try this Zipped Miranda: http://prdownloads.sourceforge.net/miranda/miranda-im-0401.zip?download
actually whole Miranda and its plugins works fine with Portable Drive but one plugin, AvatarService have some problems with it sving fullpath to avatars
Embrace your dreams
iam one of the happy user of miranda portable for the last 4month,
actually i prefer miranda to gim,trillian,