I guess he's fed up with having his name and reputation abused and misused, and being associated with an administration that would rather use him as a figurehead than pay attention to his advice.
NYT: Speculation about Powell
Wash. Post: State Deptartment Changes
Wash. Post: Powell dismisses report as 'gossip'
edit: added links
NYT: Speculation about Powell
Wash. Post: State Deptartment Changes
Wash. Post: Powell dismisses report as 'gossip'
edit: added links