Doubt there's anything to "Charles de Gaulle" in the Gulf - at least official explanation from both US and France is that "it represents our cooperative operations in support of coalition efforts in Afghanistan", "and is conducting missions in support of Operation Enduring Freedom and International Security Assistance Forces (ISAF)". I have no reason to think that it has anything to do with Iran.
de Margerie charge is somewhat murky at this moment, it might have something to do with the elections and/or, as you mentioned, prevent-defense on OFF and now Iran. With Chirac out, and Sarkozy likely to win, they might want to start with clean slate, and de Margerie would certainly be dispensable.
Hmmm, now Iran has kidnapped a bunch of British sailors who were conducting a smuggling investigation in inflatable boats. Things are definitely heating up with Iran.