Its been requested and discussed, thought folks would appreciate an update.
A new version of Trillian was just released, and it appears to now be portable.
Just run the "standard" installer, click on the "More Options" button on the path selection screen. Uncheck the Add Desktop Shortcut, Add QuickLaunch Shortcut, and Add Uninstall Information boxes. Point the path to \PortableApps. Complete the install.
I tried this with a new install as well as "upgrading" my U3 hacked version.
Its seems to work just fine. I haven't tested every feature and function, but I use it daily on 3 different machines each with different drive letters and haven't had a problem.