This reminded me of one of the stunts pulled at DEFCON 2007, someone created a honeypot stack of 8 Linksys routers to cover different channel ranges. They had the routers stacked on-top of each other, but they kept failing due to heat, so they had to put fans in-between to keep it all cool. It was on an old
TG-Daily post, but the images are gone (or i just can't see them), it just shows you that routers do indeed get hot.
One of my old routers, I had to wire in a fan to the 12v inlet to keep it working - wasn't the best idea in the world as it was a high powered fan, eventually overloaded the power brick.
I keep my routers upright now with plenty of space around them. My main is on-top of a Tower PC with a gentle breeze from the back of the PC flowing up and over it. Hiding them in a cupboard is just a bad idea. As a result, had the same router work near flawlessly for over 5 years now - whether it's due to airflow or the fact i have this ability to keep equipment going long past it used by date; is beyond me.
Would I buy a dedicated piece of molded crab-like plastic shroud for my router? Not a chance - pretty much for the same reason as hard drive cooling enclosures.... if the fan fails, you end up trapping heat instead of removing it.