Basically the White House just caved to the 9-11 investigators on this one in the face of a threat of a subpoena (and falling poll numbers…). The full Commission won't access the documents—which include the Presidential Daily Brief—instead only a sub-committee (picked by the committee) will get access. That means there's a fig leaf for the White House. But the sub-committee can share what it learns with the rest of the committee. So it's a pretty threadbare fig leaf.
The AP story gives one account of what the fuss might have been about (in addition to any administration's natural institutional reluctance to share intel with investigators), which it says is a year old, but I had never heard before:
The White House confirmed last year that one such report in August 2001, a month before the attacks, mentioned that al-Qaida might try to hijack U.S. passenger planes. National security adviser Condoleezza Rice (news – web sites) has described the report as an analysis, rather than a warning, and said hijacking was mentioned in a traditional sense, not as it was used on Sept. 11.