My guess is that this is a deliberate strategy. If the WH makes a big issue of it now, the press can't ignore it, and will have on a million liberal guests to immediately spin it, after which they will sweep it under the rug.
If Bush lets the story percolate for a week or more, lets it get legs of its own and lets people get wind of it slowly, then it is "out there" in a way that doesn't look just like a WH denial. THEN the WH can start playing to it more, with greater effect.
Just a possibility.
Cyncooper pointed out on another thread that the Plame investigation is still going on and the WH has to be careful what it says during an ongoing investigation. I had completely forgotten about that aspect of this, and in retrospect I think the WH is doing the right thing by taking a low-key approach at this time.