My Thunderbolt got way hotter than this phone. And on this phone, you can remove the back cover without disrupting anything other than the look and feel, so it can cool off a bit better (you couldn't with the TB without losing 4G and GPS).
I see it as a nonissue. It's a quad-core processor in something the size of our hand, with no fans. The fact it doesn't melt instantly is impressive enough for me.