Excellent, non-Wiki- summary of executive privilege and its uses by Presidents here:
http://legal-dictionary.thefreedictionary.com/executive+privilege
looks like it was under Ike that the privilege got to the point where Obama could claim it as he has now simply on behalf of communications between executive branch employees
though under Nixon judges chipped at a that a bit calling it “not absolute”
looks like a work in progress does it not?