Given the amount of time the candidates get at these debates and the dwindling ratings these debates garner, this strikes me as a pretty smart move:

Frustrated with the volume of interest group forums and non-party sponsored debates, Sen. Barack Obama’s campaign manager has put his foot down: Obama won’t attend any more debates that aren’t sanctioned by the Democratic National Committee, and he won’t accept any more invitations to speak at candidate forums.

In a memo the campaign will post on its website shortly, campaign manager David Plouffe writes that Obama has already spoken at 19 different candidate forums and has participated in seven full debates and is committed to attending a total of fifteen debates.

(via)
Though, he will probably tick off the special interest groups sponsoring the debate/candidate forum.