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/

ICQ Pro 2003b
Homepage ~   http://www.icq.com
Download ~ http://www.icq.com/download/

ICQPro 2003b and TrillianPro v3.1
me again ;) I have one ICQ ID running on ICQ Pro 2003b and another ICQ ID running happily under Trillian Pro

file transfer...
is super fast and works brilliantly in both directions...

ICQ Pro supports video (it's an addon available under Services > Add More Features)
but Trillian doesn't with an ICQ user....

typing notification works a treat both ways (and reliably so :)
TrillianPro detects typing on ICQPro	ICQPro detects typing from Trillian Pro

Signing on and off, privacy settings and allowing visibility to select users works a charm, the only quirk is that Trillian seems to detect ICQ users as being on AIM.... I know ICQ is owned by AOL but ICQ is still ICQ :p Overall the experience isn't earth shattering but it's super reliable, the ease of file transfer was and still is impressive but then ICQ has always had that ability to walk all over other IM's attempts to send files.

Using ICQ 5 or ICQ for Mac 3.4 in any way, shape or form was just a big no go.. for me at least it is. How strange that I could happily chat away on ICQPro to my other ICQ ID on Trillian but when I tucked ICQPro to bed and dusted off ICQ5, and subsequently ICQ 3.4, it all went to custard... that being I could happily send to my ICQ ID on Trillian but the messages never arrived nor did I receive any notification from ICQ that there was a problem.

ICQ 5 can detect that my ICQ ID on Trillian is available... ICQ 5 lets me send messages to Trillian (they just never arrive) and it even detects my Trillian avatar, I just can't have a conversation with it.

Here's where it all goes wrong...
If I have my ICQ ID logged onto Trillian and my AIM ID logged off then Trillian has no idea of the status of any ID I log into ICQ 5 but ICQ 5 knows that I have ICQ logged on under Trillian. If I log my AIM ID onto Trillian suddenly my ICQ 5 login is detected... but I can only send to ICQ 5 from my AIM ID... I wont let me send from my ICQ ID.... all very confusing.. I suspect it has something to do with how Trillian detects my ICQ friends as AIM users.... more strange is that ICQ Pro doesn't suffer the same fate.

If you intend to use Trillian to talk to your ICQ buddies... just be aware that if they are running ICQ 5 or are on their Mac using v3.4 then you may suffer the same issues... ICQ Pro is the only way to go...