Based on Trillians Unification Features Summary and Comparison chart the following features are available to us when we use TrillianPro to IM with someone using MSN Messenger...

Versions Used:
TrillianPro version 3.1
Homepage ~ http://www.ceruleanstudios.com/
Download: http://www.ceruleanstudios.com/downloads/

MSN Messenger version 7.5.0324 (Windows XP) 
Homepage ~  http://messenger.msn.com/
Download ~ http://messenger.msn.com/xp/downloadDefault.aspx

Here's me (using my MSN Passport bee@wackyb.co.nz) on Trillian Pro version 3.1... talking to myself (yep, I'm still odd ;)
on MSN Messenger version 7.5.0324 (in Beta at the time) using another MSN Passport jimjacknme@hotmail.com

I tried webcam but neither side worked....
Trillian Pro told me I (JimJacknMe) needed the latest version of MSN Messenger
which is all good... but I already had it ;)

MSN Messenger told me that I (bee@wackyb.co.nz on Trillian) was using an older
version of MSN Messenger and that the feature (webcam) wasn't supported...

Typing Notification / Status works well...
Trillian picks up I'm typing on MSN Messenger...	MSN Messenger picks up I'm typing on TrillianPro...

File Transfer works a treat!
TrillianPro has no problems sending files to MSN Messenger...	The request came through from Trillian Pro to MSN Messenger and the file downloaded perfectly...

I have no problems at all with Display Pictures, both side support this and do an excellent job of displaying and keeping them up to date no matter how rapidly you change them ;) If you opt to share your MSN Messenger background with a Trillian Pro user it's treated as a file transfer.. I'm wondering if I broke it *blush*

Voice Chat I couldn't get to work either, I suspect Norton was responsible for killing that as it appears to have treated it as a worm intrusion attempt.. ho hum yada yada.

A super cool feature of MSN Messenger is the custom smileys... sadly (but not suprisingly) these aren't supported when you're chatting with a Trillian user, add to that list Nudges and Winks.

At the moment I'm have more woes than joys :( Trillian Pro (for me) supports only the bare basics when chatting to a MSN Messenger user, the fact that file transfer worked was a pleasant surprise but I was kind of hoping webcam would kick into gear and play punch... sadly nothing I seem to do will make this must have feature work, if I get a chance I'll poke around the Trillian forums and see what pops up that might help but for now, unless I'm really desperate I wont be using Trillian to talk to my MSN buddies....

btw... I have no problem using TrillianPro with MSN Messenger for Mac version 5.1.0... file transfer is a breeze here too, sadly though MSN for Mac doesn't seem to support webcam yet.. ppffhhtt! :p