Standard procedure in Petraeus' case is to debrief him of all clearances upon resignation. He resigns NDAs at that point. He can appeal to retain some clearances if he can show need for further employment AND it is acceptable to the U.S. Government. That's the kicker here: the U.S. Government must approve for him to retain any clearances. If, as we believe, he is being forced out so as not to testify before Congress of what he knows by virtue of his classified accesses, then the U.S. Government will not make the mistake of granting him any clearances. I believe this is controlled totally by the executive branch. He would have to go public and risk prosecution for defying any NDAs he may have signed. Congress might grant him immunity for his testimony, but violation of his NDAs is a Justice Department matter.
If the Petraeus affair becomes a national security issue, like some are really trying to make it, then all bets are off regarding his testimony. He could be indicted and tried in the court of public opinion. I'd put him on suicide watch at that point (Adm Boorda comes to mind).
As for Admiral G, damn the torpedoes full speed ahead gets you torpedoed in this administration. Every Democrat President wants to be Truman vs MacArthur to demonstrate civilian authority over the military, it's in their jeans (pun intended).