The popular charge was that Rove violated Public Law 97-200 - the Intelligence Identities Protection Act of 1982. <
http://caselaw.lp.findlaw.com/casecode/uscodes/50/chapters/15/subchapters/iv/sections/section_421.html>
For these charges to stick, more than one elements of proof would have to be substantiated:
1. Plame was a person under CIA cover - probably true.
2. Plame had been posted outside the US within the last 5 years - probably not true.
3. The CIA had to be taking measures to protect her identity - probably true.
4. Rove had to know #1 and #3, above as a result of lawful access to classified information. Highly unlikely since Rove was not in a National Security Position and even if he had access to intelligence product, the CIA would not have shared "sources and methods" (like identities) information with him. Even if Rove had been told that Wilson was related to a CIA Analyst working Counter Proliferation (CP), a reasonable person without specific access makes no apparent nexus from CP to Covert Officer - which she had been in the past, albiet not posted overseas within the prerequisite five years.
5. Rove had to convey said information intentionally to an individual not authorized access. Great question since the record suggest that it was the press (Cooper) that disclosed it to him.
But, all the elements of proof must be met. You don't get a lesser convistion for trying hard with 3 of 5.
When the case can't be made here, the next step is to examine if he misled the FBI or committed purjury to the grand jury.
Now, the question of the sealed indictments - consider the possibility of one or two people at CIA, who were in a position to know that Plame's identity was classified (even if not qualifying for PL 97-200. They explain to an enraged White House just how all this happed with Wilson's trip and all, and disclose Plame's identity. They have access to her classified (if not currently covert) status but disclose it without properly safeguarding the information by making sure the recipient knows that its classified - or they spaz when the FBI show up to question them and now face charges. However, THEIR names are still classified, so the indictments are sealed.