It is not (yet) optimized for the new MBAs. Wait a little. Normally, when the temperature reaches more than 44°C it will show correct values. You can also dig a little in the source code. I manually added the new Macbook Air 6,1 to the Machines.plist, for example, and recompiled the project. Works great so far.
You can also use istat menu, but it'll cost you 16 bucks.