This happened to me exactly twice since having the MBA, which is about nine months. A reboot fixed it each time. I don't know why it happened and it hasn't happened since.
If you just installed the OS X 10.7.4 update it's normal for it to get a bit hot as it installs. This should last no more than a few minutes.
Run activity monitor, and see if Adobe Shockwave is running or not. I had Chrome running with 12 tabs, MS Word, MS Excel, VMWare Fusion with Windows 7 running with Outlook running. Fan was making continous noise. I killed the Shockwave process, and then went through the Chrome tabs - found the tab that was complaining about Shockwave plug in failure, and closed that tab. The fan went very quite in less than a minute!!!!