The point of the investigation isn't to discover who printed the story (that would only take a buck to buy the paper and a few minutes to read it). The point of the investigation is to find out who in the government leaked the information, and I don't see why the White House would be considered off limits.
NO one is off limits in the investigation. However, some folks have more to gain than others in leaking this top secret information. This leak does not advantage the goals of the White House but it does further the cause of the anti-war, anti-Bush side. That side includes some Republicans, but statistically, it is more likely that Democrats are behind the leak.
This 'Reach into the White House' line is so much boilerplate from the dems. They're desperate and they should be.