Is it an intel or an AMD? If its an intel, don't worry about it, the worst that will happen if it gets too hot, is it will crash. If its an AMD on the other hand, it could just go pop haha (seriously though, very very unlikely that would happen).
On my old P4 laptop, I forgot to clean it for a few months it see's more use in a month than most peoples do in a year) and had been using it in bed a lot. The fan on that thing (Pentium 4 Northwood - "not" a mobile CPU, so extremely hot anyway) could suck better than a pro, and eventually it started crashing when it was on for 20 minutes or so.
When I eventually could be arsed to clean the heatsink and pipes, the dust a like a persian fookin rug!!
On this lappy (Core2Duo, very cool running anyway), I only ever run in a eco-mode (1.00Ghz) as thats still more than fast enough for business use, and the last time I cleaned it (after 9 months non-stop usage), there was feck all dust there haha!
Get some air-spray and blow around the fan (through both the intake and extractor vents) and see if anything comes out, or it makes a difference. If not, your gonna have to open it up and see if its clogged with dust - not usually a hard job believe it or not, the CPU's are quite accessible most of the time - but if in doubt, I can take a look at it for you. Will only take a few mins to see if its easily accessible to be cleaned and do it if so.