If you are looking for a "spirit of the rules" which was broken, see Obama's national campaign ad that ran just prior to the Fla. primary - this DID violate the pledge.
b. A presidential candidate who campaigns in a state where the state party is in violation of the timing provisions of these rules, or where a primary or caucus is set by a state’s government on a date that violates the timing provisions of these rules, may not receive pledged delegates or delegate votes from that state. Candidates may, however, campaign in such a state after the primary or caucus that violates these rules.
“Campaigning” for purposes of this section includes, but is not limited to, purchasing print, internet, or electronic advertising that reaches a significant percentage of the voters in the aforementioned state; hiring campaign workers; opening an office; making public appearances; holding news conferences; coordinating volunteer activities; sending mail, other than fundraising requests that are also sent to potential donors in other states; using paid or volunteer phoners or automated calls to contact voters; sending emails or establishing a website specific to that state; holding events to which Democratic voters are invited; attending events sponsored by state or local Democratic organizations; or paying for campaign materials to be used in such a state. The Rules and Bylaws Committee will determine whether candidate activities are covered by this section.
http://www.gwu.edu/~action/2008/chrnothp08/sanctions.htmlObama made a political calculation and snubbed the voters of Michigan - there is no other justification for taking his name off the ballot.