I had the same thing happend to me, just once. (mbpr 15", 2.3Ghz, purchased mid december 2012)
I had left it with the charger plugged in and lid closed all day, and when i got home and was going to use it, it was much hotter than usual (couldn't touch the underside for more than an instant, middle part where the cpu/gpu/ram is - check this picture for reference. http://www.google.no/imgres?um=1&hl=no&safe=off&tbo=d&biw=1536&bih=761&tbm=isch& tbnid=RWqPK2XLiWMn6M:&imgrefurl=http://forum.notebookreview.com/apple-mac-os-x/6 69903-official-macbook-pro-2012-thread-19.html&docid=L7EwbBxKHScfsM&imgurl=http: //www.extremetech.com/wp-content/uploads/2012/06/macbook-pro-retina-display-inna rds-labelled.jpg&w=878&h=604&ei=jOboUOjfAeKN4ASpqID4Ag&zoom=1&iact=hc&vpx=284&vp y=281&dur=626&hovh=187&hovw=272&tx=182&ty=83&sig=106963892494486786340&page=1&tb nh=147&tbnw=214&start=0&ndsp=33&ved=1t:429,r:9,s:0,i:111 )
I had done the same thing as OP, closed the lid so the mbpr was suspended, nothing was running (had left a paused video in vlc, and safari open with only 1 tab containing some news site, a paralells win7 vm (the bootcamp partition) had an active window, but was powered off - just like i've done dozens of times).
When i opened the lid it took abormally long to get started 10-15s (while waiting a swirly-multicolor circle replaced the cursor), and the fans went to maximum for a minute or so before it was cooled to normal temps.
While it was doing this, i instantly checked top, load was low (all three values, so it couldn't have had high cpu load for 15 min+) , no processes doing anything special.
