It is possible to clean the inside, you just have to take the upper case off and use an air blower to get the dust out (use extreme caution when taking it apart and blowing it out, you would rather have a constant fan than a blown logic board!)

However, my fianceé's macbook fan was always on, and the computer ran extremely slow, and it seemed to me that that she had a software issue and not a dust issue. I used time machine to back-up all of her files/applications, and then erased her HDD and gave her a fresh install of snowleopard (she had regular leopard before). This fixed her fan issue as well as the slow computer issue. Im not sure if you have this problem as well, but if you're computer is running a lot slower than it should, I would think of trying this idea before taking the computer apart.

Hi! I would try resetting the PMU, i.e. with the power off, hold down the power button for about 10 seconds until you hear a tone and see the sleep light flash. This restores many of the laptop's core functions to default values, and I've witnessed erratic fans start behaving better afterward.

I had a similar problem, the cause ofwhich was software related and the printer driver I had recently installed was stuck in a loop. After removing said driver normal service was resumed.

It may be worth looking at any recent software upgrades or instillations before taking things apart.

I'm a big fan of iStat Menu for monitoring my computers temperature. When I get an anormal temperature raise I open Activity Monitor located in -> Applications -> Utilities folder to check what's going on and which application is the cause of the temperature raise.

http://www.islayer.com/apps/istatmenus/