Hell, if bush can somehow keep mccain in line behind him I imagine they certainly have numerous ways to keep powell in check too.
For one, if they wanted to dredge up old vietnam issues to use against him, one of powell's first big tasks as a military commander was being a part of the operation to keep the mai-lai massacre and other vietnamese attrocities under wraps. Perhaps its the case that Powell has always had a penchant for being the government's lying mouthpiece since it has assisted his rising through the ranks.
In fact, for those that are curious how powell got to the position that he is today you might want to check out the following reading. Knowing his background greatly reduces my ability to have any respect for the man IMHO.
http://www.tbns.net/mediapoet/pros29a.htm
http://www.freerepublic.com/forum/a3a3ff03229a4.htm