Like many programs flash their window on the taskbar / dock to alert the user to switch to the program,
Is it possible to flash the
At this point, it seems only causing an alert dialog to pop up does the trick... this seems a bit too intrusive, I feel, particularly given the use you're trying to put it to. Instead of causing it to flash, though, you could modify document.title to grab the user's attention, either by prepending some signal (perhaps the string "NEW!") to the site's name, and then using an interval to constantly change it to "", which would then give a nice little "flashing" illusion.
Bare-bones example:
Chat System