As long as it's not actually overheating or getting too hot to touch, this is pretty normal behavior. I'd download the iStat Pro widget to keep track of the temperature, if you haven't already. But YouTube takes a lot of resources and will generally cause a laptop to get hotter than usual, so it's not necessarily anything to worry about. Make sure it's sitting on a surface that is not suffocating the vents (don't put it on a blanket, pillow, couch, etc.) Also, I assume with this model you've probably got at least 4GB RAM, but if not, you might consider expanding, because having plenty of RAM can help. You might also try the standard resets (PRAM, PMU/SMC) because they can help normalize the power settings if they've gotten out of whack.