How many times You missed a trade just because you've been hanging around the iNet (not in d2jsp) and missed the Pms from the other traders?
or just watched the TV next to your computer?
well im sure it happens all the time... So
my suggestion is making an Option that enables you to get PM sound alarm (like Sms) when you get one... Even if not refreshed.
Obviously if you are not interested in that you may Disable it.. So either way it cant bother you..

Just leave the Instant Messenger open in another tab
It makes a sound when people post in your topics and when you get PM'd