Same issue here on an i5 MacBook Air 11" which is otherwise running great.
* On a cold boot it stays cool while idle even when it's been on for hours. Also, it doesn't get unusually warm even after extended periods of heavy use.
* If I unplug the Air, put it to sleep for a few hours, wake it from deep hibernation and then plug it in after I've woken it up temperatures remain perfectly normal.
* However, If it's hibernating and I plug it in before I wake it up temperatures begin to soar without any visible CPU load in the activity monitor and without any taxing apps running. Fans run full blast and it gets noticeably warm to the touch after a while.
* If it's running hot after waking it from sleep a reboot or putting it back to sleep and then waking it up again always seems to fix the issue and temperatures quickly return to normal.
Tried SMC reset, PRAM reset etc with no luck. No other issues whatsoever with it other than this.
Cheers