Is it normal for a 15 inch rMBP to get extremely hot during use?
Your computer is designed to run all 4 cores and generate heat like a 60 w lightbulb would. Internally, the CPU doesn't get as hot as a filament does - but it does approach the boiling point of water before it will slow itself down to avoid overheating.
This control is built into the chips themselves so even if your fans stop working, the machine should run as fast as it can given the amount of heat that escapes through the warm aluminum frame. The heat that you have noticed may simply be the system working as designed with passive cooling assisting the fans.
Here is an article explaining a bit on how you can measure the internal temperatures, how the loading of work on the CPU spins up (and even sound recordings of) the fans:
- http://www.anandtech.com/show/6023/the-nextgen-macbook-pro-with-retina-display-review/12
My guess is your mac is running normal as it can be quite uncomfortably hot and still operating as designed. However, if you are worried, you should call AppleCare and ask them for assistance in determining if it's running safely. Rather than trust that our understanding of how hot your Mac is since you could be someone who is underestimating danger, or another that over estimates it and I'd hate to not have you get a second opinion or know exactly how hot the internals and case are. Consulting the manufacturer would be a great next step for you.
The unit probably gets hot from pushing all those pixels with the discrete graphics card. If a mac gets too hot it will shut down before damage is done. I believe that functionality is built into intel CPUS. Forum post about shutdown temps.
Have you tried cleaning your mac out with canned air? It might be a bit dirty inside.
Mine gets plenty warm when I am using it for anything more than browsing the web or email. Actually, even when that is all i am doing, it doesn't exactly stay cool.
As mentioned in other answers, Intel CPUs do have excellent thermal protection built into them (unlike some other CPUs). There are videos on YouTube showing CPUs and how they behave when their heatsink was removed - don't try this at home, kids! One popular brand let out some of the magic smoke and died. The Intel one just got slow.