We know that the supervolcano that IS Yellowstone - its caldera is vast - erupts on the average every 600,000 years. We are overdue at present for another eruption.
If you enjoy fiction based on fact, and a good disaster flick now and then, I recommend Supervolcano – It’s beneath Yellowstone and it’s going to blow. I discovered this little made-by the BBC gem this summer and have since purchased the DVD. It's based on good science and, in my opinion, a better romp than Dante's Peak if for no other reason than it IS going to happen. It's just a matter of time.
If it gives you a case of the heeby-jeebies, you can always console yourself with a copy of my book, A Little Guide for Your Last Days. Cheers!