Should I be worried about 98°C CPU temperature on MacbookAir?
Michiel's correct in that if the computer gets too hot, it'll shut down automatically. However, if you want to be on the safe side, you can check out smcFanControl and Fan Control.

smcFanControl:


  smcFanControl lets the user set the minimum speed of the build in fans. So you can increase your minimum fan speed to make your Intel Mac run cooler. However in order not to damage your machines smcFanControl doesn't let you set a minimum speed to a value below Apple's defaults (MacUpdate).


Fan Control


  Some MacBooks and MacBook Pros are just to hot for daylong use. Fan Control adjusts the minimum fan speed depending on the current CPU temperature. For safety, it leaves the original automatic fan speed control intact (lobotomo).


And remember, if your computer ever gets to 660.4°C, don't leave it on your lap, unless you want liquid aluminum all over you.

UPDATE:

I found another tool called CoolBook that


  lets you monitor the frequency, voltage and temperature of the CoreDuo / Core2Duo CPU. Registered users can also change the frequency and voltage of the processor.


Here's a list of features that are listed on the site:


Reduce heat (presumably by optimizing fan speed)
Reduce fan speed (presumably by lowering fan speed)
Extend battery life
Improve performance
Fine tune your CPU


This seems like a useful tool, but it does not support Lion and Mountain Lion, and does not work on Intel i3, i5, and i7 processors.