CPU Die temperatures reaches 90° C, is that normal?
The MacBook Air 13", Mid 2011, comes with an Intel Core i5 or Core i7 (2557M, 2677M) ("Sandy Bridge") Processor and the maximum temperature allowed at the die is 100° C, per Intel's documentation on both processors.  (See TJUNCTION in the links provided.)

So while you have not yet max it out, nonetheless it shouldn't be running that hot except under considerable load. If you'er seeing that high of a temperature under an idle load then it is something to worry about.