Help, I'm Stepping Into This Twilight Zone
When you've got one of the most secretive administrations in history, it's hard not to turn to conspiracy theories to explain the unexplainable. Here's an interesting and creative stab at why George Tenet left, thanks to The Fray at Slate:

First, there's the possibility that Tenet himself was the "leaker" in the Plame affair, and Bush just found out. This inspired him to call a lawyer to see how best to protect himself and the Administration. . . . and the lawyer told him to fire the dude ON THE SPOT.

Second, and more interesting to me, is the possibility that Tenet wasn't the leaker, but that both him AND Bush just found out who was, and it was somebody so highly placed that Tenet had to (immediately) resign, and Bush had to (immediately) call a lawyer, to figure out what his obligations are, now that he knows.

And as I type this, I just heard Corey Flintoff (my fave NPR newsreader) announce that another CIA top dog (James Pavitt, director of operations and in charge of spies like Valerie Plame) is announcing his retirement tomorrow (though it's being characterized as being planned well in advance of Tenet's announcement). Sure.

Speaking of Slate, Fred Kaplan has a good wrap-up of the announcement and the week for the Bush administration.

And here's Josh Marshall's take on the week that's been (and isn't over yet):

Having said all that, beside the possibility that the White House's favored Iraqi exile was an Iranian agent, that the spy chief just got canned, that the OSD is wired to polygraphs, and that the president has had to retain outside counsel in the investigation into which members of his staff burned one of the country's own spies, I'd say the place is being run like a pretty well-oiled machine.