Specifically, that he's about to become a lobbyist, which would therefore take him out of the running for any position within the Obama administration, since it has ethical guidelines against bringing in (most) lobbyists aboard.
The HuffPost's Sam Stein notes the misinformation began with sloppy reporting by the WashPost Al Kamen, who didn't try to confirm his report that Dean would be lobbying for the D.C. firm McKenna, Long & Aldridge. Kamen's claim was then picked up and amplified by USA Today.
As Dean's spokesman told Stein:
"[Washington Post reporter Al] Kamen got it wrong this morning," said Karen Finney, a spokesman for Dean. "He wrote that without talking to me or Dean. As you know, Dean is an Independent Consultant for the firm and not a registered lobbyist."
Note that the Post's Dean report was built around the flimsy phrase "sounds as though," and USA Today's was built around the equally flimsy "seems." (i.e. This seems to be the official end to Howard Dean's stated hopes of joining the Obama administration.")