If you are on Mac OS X or Ubuntu, the problem is caused by the symlinks to the JDK. File | Invalidate Caches should help. If it doesn't, specify the JDK path to the direct JDK Home folder, not a symlink.
Invalidate Caches menu item is available under IntelliJ IDEA File menu.
Direct JDK path after the recent Apple Java update is:
/System/Library/Java/JavaVirtualMachines/1.6.0.jdk/Contents/Home
In IDEA you can configure the new JSDK in File | Project Structure, select SDKs on the left, then press [+] button, then specify the above JDK home path, you should get something like this:
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