and Flash on a Mac will cause your machine to get really hot.
However, the fans should not go off when that happens. Given that the machine is still under warranty, take it to an Apple store and tell them about your problem.
I have a MBP about 6 months old that I recently figure out why it is getting so hot and the fans turn off, usually when I am watching some videos online. Like a news video on CBS.com. It used to get hot (the underside) but not this hot so since I could not get advice on the apple forums (I am not even sure what happened to the old Apple Forums). what happens is the fans shut down and I turn the machine off too worried to see what will happen next. I do know where I teach has a lot of dust and lint and I can just imagine how the inside needs to be cleaned out with compressed air. is this something the genius bar needs to take care of, will they if I bring it in, or can I do it on my own with greet care?