I've been researching all day to find out why my macbook is overheating so regularly and I found a few common solutions. This thread has a lot of good information, so I'd encourage everyone to read all the way through it. I'll sum up what information I've gathered to help the next person reading this thread diagnose their problem(s).
1. Download smcFanControl first and foremost, for two reasons:
-One, is that you can easily monitor temperature and fan speed, and having these measurements will allow you to gauge whether the next steps you take to reduce overheating are actually working. Once installed and opened, you will see the measurements up on the menu bar at the top of your display.
-Two, is that you can adjust fan speed which may reduce overheating.
2. Energy Saver Preferences / Video Cards - are you running on 'higher performance' or 'better battery life?' I found that when I switched to 'better battery life' my computer is much, much cooler; it makes sense. I have a 2009 15" Macbook Pro with two video cards, a 2400M and 2600M GT, so switching preferences switches which video card I run on. You can see if you have more than one chip by going to your hardware specs and then the graphics/display specs. I'm not sure what this says about my 2600M GT card... it's never done this in the past so perhaps I burnt it out or something.
3. Activity Monitor - how much of your memory is being eaten up and by what processes? Perhaps you have a program that is eating up all of your processing capability thus heating up your computer. Activity Monitor gives you a list of processes and what percentage of your memory they are using, so perhaps you can diagnose what program is causing your computer to overheat. You can find it in your utilities folder. Drag it onto your dock so you can have a live status of CPU usage. Once open, there is a nice little graph along with some quick numbers at the bottom to give you a picture of memory usage. In my list of processes, I found that iTunes would occassionally glitch out and hog a ton of my CPU, so when I see those bars go up on my dock I restart the program. I also found that Dropbox was using a little more memory than it should have been and I changed its settings so it doesn't open automatically on startup. My understanding is that if it's not an application that is eating up CPU, and it's some natural process like kernel_task, WindowServer, or mdworker, then you don't want to mess with it.
4. Disable Flash - Running flash videos, games, or whatever inevitably requires a lot of processing power. Apparently, Macs and Adobe Flash don't work too well together. The only time I really use flash is when I watch YouTube videos, so I disabled flash and enabled html5 video playing for the site. You can get the add-on for Firefox here; check the 'disable flash' box in the preferences. If you want the choice to disable flash on any website, download FlashBlock. And on a side note, if you run Firefox, you must have Adblock Plus.
If none of that works, I'm not sure what I would recommend. Resetting the system management controller might do the trick, or running a hardware test might give you some indication of the problem. I don't know too much about that stuff though, or how helpful it might be. Maybe you need to clear the dust from your computer's cooling system? Or maybe the geniuses can help.