Though this seems like a good idea, I don't know if it's possible. I could be wrong though.

If someone is online, and you send then an IM, the very next action they take, wether it be to reply to a message, go to a different message, or whatever else, the message at the top of the screen will always show the "Hey, YOURNAME, you have X messages, XX are new." So, if they keep an eye out at the top of their screen, they will see when they get a new IM. Or, you could apply a mod similar to what I have *I can't remember where I picked it up* that pops up a little alert box when someone gets a new IM. That way, they're bound to not miss it.
