I was worried about running Raspberry Pi in summer, hot weather is not really friendly with electronics. Worried not enough to put it in 24/7 air-conditioned room, but somewhere around spending $5-8 on heatsink installation, using heatsink kit from ModMyPi.com
The kit comes as three separate radiators, one for the main CPU/memory unit, one for the periferals chip responsible for ethernet and usb, and one really small one — for the power converter, that also gets quite hot.
Installation was a breeze: just attach sticky heat-transfer tape to the bottom of the heatsink, cut to the size and stick the heatsink on the top of the chip. And maybe move around a little, to ensure the perfect fit and there are no air pockets trapped between the tape and chip surface.
The results. I could not notice a very sharp decline in the temperature graph, but still cutting a few degrees off the CPU temperature might prolong the life of the Raspberry Pi for a few more month, if not years. The active cooling system with large fans and radiators might give a somewhat better results, but the price and the noise will definitely be much higher.