I am realy stumped by this. I just noticed that I had a new PM, but no flashing indicator. That's on our board running 7.5.

So that gave me the chance to do some testing.
tried refreshing the page several times -> no luck
cleared the cache -> no luck
logged off and on again -> no luck
closed the browser (FF 3.0.6) and restarted it -> no luck

So I started another instance of FF to check some data in the database directly but couldn't find anything wrong there.

Then, while still in my second FF-window, I open the forums and tadaa there is the flashing indicator.

Now back to the first window, press F5 and tadaa a flashing indicator here too.

I haven't got a clue where to search in order to tackle this problem, but maybe it gives someone else an idea....