Mine hasn't been hot yet, but my X and OG would sometimes. I suspect the bionic will too. I get crap done fast on this phone lol. Bonus for battery life.
Man, my screen gets pretty hot if I am doing anything for more than 10 minutes or so. This is especially noticeable when I am charging it while using it... but my extended battery doesn't get beyond mildly warm. I expect this when you have a dual core 1GHz processor running.
My phone gets warm when running apps and doing stuff for a while, it happens. The thing im worried about it when im playing Star Legends or a game for a while (~10-15 mins) the screen itself gets REALLY hot, like to hot to touch for more than 10 seconds.
My Bionic(with extended Battery) seems to run pretty hot as well. While it doesn't seem to be a huge problem inside(or with the cool weather we've been having in the past two days), if I'm out on my walk it gets really warm, really fast.