I just found this thread, it's very interesting to remind these things. I happen to work as a IT professional, so if I'm allowed to add a few words of advise about this...
Desktop towers, CPU fans, heat sinks, etc, have improved greatly in recent years, allowing for more and better air circulation that ensures the proper cooling that state of the art computer systems need. Unfortunately, the fact is that the more air that circulates through the tower, the more dust it will gather, therefore my advise is (at the very least once a year) to open the tower and clean inside as thoroughly as you can, particularly the areas near where the tower, power source, graphics card and CPU fans are located. Bear in mind though that the electronic components around there are delicate, I advise to be very gentle, there are dedicated cleaning kits around, yet with a bit of care a portable vacuum cleaner (or even a big one with of those thin accesories made to clean difficult areas attached) can do the trick, it is not necessary to leave the motherboard impecable (you can do more bad than good trying that) for it is of no concern if a thin layer of dust covers it. It is crucial though to ensure than dust does not prevent air to circulate properly, particularly through the CPU and graphics card heat sinks, not only because excesive heat can damage them, but also because an unproper ventilation also causes the rest of the system to overheat (state of the art CPUs and GPUs can generate an awful lot heat), contributing, for example, to a hard disk failure.
Moreover, I strongly recomend to, once a year as well, remove the CPU heat sink and clean it well: Use a vacuum cleaner to get off all the dust you can from the heat sink, if theres's dust stuck between the sink's fins you may use a thin knife or similar to take it off (NEVER DO THIS WITH THE HEAT SINK ATTACHED). Also, with a cotton cloth, gently but thoroughly remove the thermal paste from the top of the CPU core and the bottom of the heat sink, replace the thermal paste on top of the CPU core (cover the core completely and generously, but just the core since it's the area that will generate the heat and the paste will expand through the surounding area when the heat sink is replaced on top of it anyway). This also applies to higher end graphics card if you own one. It takes five minute, thermal paste is cheap enough and you will ensure a longer life to your systems, remember, dust stuck between the fins of the heat sink means that no air circulates through them, dramaticaly degrading it's effectiveness.
One last piece of advice, all this also applies for portables, even if mobile CPUs and GPU are made to consume less power and therefore generate less heat, it is also true that their ventilation systems are less powerful (no use in attaching to a portable CPU one of those heat sinks that we find in desktop towers which look kind of like a motorbike cilinder
), usually the processor and it's cooling system are accessed easily enough (I own a Toshiba M40 and it's a piece of cake, just opening a small lid with a philip screwdriver). In the case of portable computer not only you'll be saving your rather expensive system from and overheat failure, but also you'll save power and therefore battery just because air circulates better when everything is nice and clean and therefore the cooling fan will not need to operate at full speed all the time.