I'm sorry for my delayed reaction.
My MSN version is 7.5.0299
Messenger Plus! 3.61.145
Windows XP Pro - SP 2 - Dutch
I don't use graphical window optimisers like windowblinds or such things. The windows UI does his job perfectly I think
Strange that you both can't re-produce this "bug", because I have it all the time. A step by step guide how this thing appears.
*** UPDATE: !! I've noticed it only appears when you've maximized the conversation windows. !! ***
1. Locate your messenger main screen on the first monitor
2. Open a conversation with someone. Move this window to a monitor on your left or right and maximize it.
3. Go back to the messenger contactlist (which is still on your first monitor)
4. Open another conversation, and move this window to a monitor which doesn't contain your first conversation. (maximize it!)
5. Use the MSN tab function, the conversations will remember their positions, so with every tab you have to look on a different monitor.
As I said before, it isn't a real bug but more a practical problem.