I have a 2012 15" MBP (Non-Retina) that I use in clamshell mode most of the time. I've been doing some video encoding for a project I'm working on and was worried about head since when I'm encoding all processors are running at 100% and the fans are going full blast. I purchased iStat Menus to monitor the temperature and I found that while encoding with the screen fully open and processors running at 100% I was running at a steady 117ºF… while encoding with the screen closed and processors running at 100% I am running at a steady 122ºF. So, yes, it's running a bit warmer but that could probably be attributed to the fact that the GPU is powering a 27" Thunderbolt Display instead of the built-in 15" display.
My feeling is basically that Apple has built the clamshell-mode functionality into the machine and I don't think they would have done that if it was going to cause problems. I know that there are a lot of people who run their MBP's in clamshell mode (myself included) and I have yet to meet anyone who had encountered a problem doing this.
I've run a plastic '13-inch, Early 2009 MacBook' since 2009 almost exclusively in clamshell mode. She's still going strong. – boehj – 2011-09-04T04:15:43.933
Great, thanks for the quick and precise answer! :) – Kevin Yap – 2009-12-27T23:01:09.507
Let me add my 2c-worth to the above sample-of-one. I also run a 17-incher in clamshell mode and it works fine with Cinema display attached. Guess for the definitive answer, download istat menus and see if there's any significant temperature difference on the various parts of the machine, between operating modes. http://www.islayer.com/apps/istatmenus/
– Thomas Browne – 2009-12-27T23:57:22.497