Recieving IM causes screen not to turn off

Not sure if this is normal, but when I am on Mobile IM and have the screen locked and turned off... when I receive a txt message, the screen turns itself on and the backlight will not turn off until I read the message. I cannot even manually hit the lock button to make the screen turn off until I read the IM. Txt messages are fine, but is this normal for anyone else?

It just kind of sucks when you get a mobile IM, can't read it for a couple hours cuz you are driving or whatever, and your backlight has been on the whole time...

Thank you