I've had similar thoughts. Reporters know who Plame is from gossip, whatever, they can't go to press with that so they call the WH etc. and ask leading questions. (Andrea Mitchell admitted it was known in the political/cocktail circles who Plame was, where she worked etc.)
WH person(s) confirm who wifey is, where she works and the reporters can go to press with "White House officials" confirm/indicated etc.
The fact that Miller didn't take advantage of Rove's blanket release indicates Miller must have another source.
I think you are exactlly right. I think Plame's job and her marriage to Joe Wilson was so well known that it was just floating around journo and Administration circles for months. Fitzgerald is probably not going to be able to pinpoint a single source for a "leak" of Plame's identity. Everyone is telling the grand jury they heard it from another journo first.
We also know that Joe Wilson is a fabulist who tells tall tales and has an extremely high opinion of himself and his wife. It's very plausible that he himself blabbed about Valerie while bragging to some journos at a cocktail party about how well connected he is and about all those danegrous missions he went on to Niger. Journos probably won't even tell the grand jury they just overheard Joe Wilson blabbing at a cocktail party and started gossiping to their friends.