This behaviour has been discussed in a previous thread:
http://talk.maemo.org/showthread.php?p=682137

The problem lies in the fact that you are the last person to send an IM from the computer client and it is specific to the newer firmware PR1.2.

For more details see the filed bug report:
https://bugs.maemo.org/show_bug.cgi?id=10319
Lets hope it will get fixed on the next update.

A workaround, but not really a solution, is to open the conversation from Conversations->New IM->"The contact the you had the conversation with" and send a last message from the N900.