How hot is too hot for my macbook? I have a macbook (2.26 GHz Intel Core 2 Duo upgraded to 8GB 1067 MHz DDR3 RAM) running OS X version 10.6.8. I do a lot of prototyping for high performance computing on this macbook (running c++, fortran, mathematica, matlab, etc) and often work in parallel.

I have noticed that my computer gets hot so I have installed 'Temperature Monitor' and find that, when running difficult jobs, it can break into the 90 degree (C) range. I know this is not good, but is this incredibly dangerous? after seeing what values should i begin to get worried?

Secondly, I would like to clean (and maybe even replace?) the fan to make sure I am getting the best ventilation possible. Will a can of compressed air do the trick? how expensive/worthwhile would it be to open up the case and replace the fan?

Any help would be great - thanks!
90˚c is typical for that system under high load; anything > than 95˚c is when you might want to be concerned. If the CPU gets too hot, or starts to overheat, the system (typically) will clock down the CPU to prevent overheating. The fans shouldn't wear out, so I don't really see any point in trying to replace what's not broken. Intel Mac's run hot, and the fact that the only fan is an output fan doesn't help with heat dissipation. 

Something you could do to help alleviate some of the heat is to download smcFanControl (free) and adjust your fans RPM. Instead of the standard 1800 RPM set it at 3500-3600 RPM, which should help cool down the temp possibly 2-10˚c. You might notice more noise from the higher RPM, although you shouldn't worry about it — MacBooks fans generally max out at around 6,000 RPM, and I've cranked one up to 8,700 RPM before.