I also have my tv under the cabinet of my tv, here's the important thing, leave some space where the hot air comes out, also yuo can feel the top of your ps3, normally it never go hot at the top, just a lot of hot air coming from the side, but the top of ps3 is always cool

beside, from what I hear, ps3 has some heat detection that it will turn off automatically if it overheat before any further damage can happen, if that happen you can just wait till it cool again and it will turn on normally again