I'm running 10.8.2 and began having this problem just a few days ago, and only while using one specific user account (there are six users on the machine).
Tried many different suggestions including the keychain, date/time, and var solutions that seem to work for so many. But no luck.
Finally here is what worked for me.
In
System Preferences > Security & Privacy > General
I already had the "Allow from" section set to "Mac App Store and identified developers".
I clicked the lock icon (lower left) to make changes and entered my password.
Next I selected the "Mac App Store" setting.
Then clicked the lock to prevent further changes.
Then clicked the lock to make changes and entered password again.
Selected "Mac App Store and identified developers" --- same as it was originally.
Clicked the lock to prevent further changes.
Launched App Store and clicked on Updates.
It worked. Apparently by toggling this setting something refreshed or reset and all became well.
Hope this helps everyone else who is still fishing for a solution!