There has been a lot of fuss lately over Obama opting out the publicly financed campaign system and some of it is justified given the current lack of anti-Obama 527s. However the most likely reason for his decision is this...
From the LA Times' Top of the Ticket
McCain may lag in money, but RNC outraises the DNC by 5 times
Led by chairman Mike Duncan, the Republican National Committee ended May with 13 times more money in the bank than its Democratic counterpart, and raised five times as much money in the same time frame.
As The Times' campaign finance guru Dan Morain points out, the sums are significant as presumptive Republican nominee Sen. John McCain squares off against the far more richly-funded Sen. Barack Obama for the last 136 days of the general election campaign.
Based on the numbers so far, the Republican Party appears poised to act as the financial equalizer in the fall campaign. The RNC disclosed that it ended May with $53.5 million in the bank, compared to $3.9 million for the Democratic National Committee, which is headed by Howard Dean.
So had Obama opted into the publicly financed campaign system he would have hamstrung himself financially. So while there is much to be said for keeping your word there is something to be said for realizing the facts of the situation and adjusting your course accordingly. Literature is rife with characters that stick to their word even when its to their detriment. They are usually referred to as tragic heroes and generally speaking make for a better read than a president.
h/t to TMV