They do get warm, but not all the time. If you were burning a CD/DVD for example, you wouldn't have it on your lap for the duration, well I wouldn't anyway, so If that's the case then it wont matter that much. In normal use, Surfing, using Word/Exel and touching up the web site I never notice the heat much, although I always warn the lad when he uses it because he usually has shorts on.
I have the 15" and love it. I was also going to go for the 17" but as I'm always on the go I decided to be sensible for once.