Most politicians don't look beyond the next election. It's hard to believe that anything in politics is permanent...
This [link|http://en.wikipedia.org/wiki/Potential_Democratic_candidates_in_the_2008_U.S._presidential_election|Wikipedia] article says he would have to resign as DNC chair to run. I can't find anything in the [link|http://a9.g.akamai.net/7/9/8082/v001/democratic1.download.akamai.com/8082/pdfs/20060119_charter.pdf|DNC Bylaws] (26 page .pdf) that would indicate it's permanent. Article 5 on the National Chair says he must be impartial (Section 4). But I just skimmed it (it doesn't seem to be searchable).
Cheers,
Scott.