Word from Cline at WebmasterWorld that if you are using both Find and Edit Max CPCs Tool and Position Preference is enabled, the Edit Max CPCs Tool will not update your bids. He noticed this when the tool was "failing to put into effect the bid changes." So he asked his AdWords representative, who responded that this was "indeed a technical problem" with the Google AdWords system.
Forum discussion at WebmasterWorld.
(conference coverage, missing the first session, have a meeting in 10 minutes, will catch the next sessions, but I am also skipping the last session, so I can get home at a reasonable hour tonight).