I'm a firm believer in not assigning the term 'flip-flop' to someone unless they go back and forth on it more than once, with a clear sense that they're talking out of both sides of their mouth to different audiences. A change of mind is not a flip-flop, despite the popular tendency to use that term for a principled change of mind.
Nevertheless, there are reasons to criticize someone for changing their mind. One such reason is if they insist that they haven't changed their mind. Barack Obama seems to be in that category when it comes to gun control. He supported the DC gun ban back in February when the issue was before the Supreme Court. Now that the case is decided and the law deemed unconstitutional, he says he supports the decision. See Booker Rising for the video clips.
His only justification for supporting Justice Scalia's opinion overturning the ban now seems to be the same justification he gave for supporting the ban earlier, which is more than a little puzzling. He supports an individual right to gun ownership but thinks it's constitutional to restrict that right in certain contexts, and his support for the most restrictive ban on gun control in the country showed that he thinks these restrictions on the individual right can be pretty extensive without being unconstitutional. So the DC ban was ok. But now he responds to the opinion by saying that the same position he's held all along can support this opinion. He believes in an individual right but thinks restrictions of that right are constitutional. So the DC ban is not ok. Yet he's insisted that this isn't a change in his view. How is it not a change of view to think it's unconstitutional but then to agree with the Supreme Court's narrowly-divided opinion for the opposite view?
So I have a hard time distinguishing this from the mindset of flip-flopping, even though it's not a case of going back and forth. It's what you say on a divisive issue if you want everyone to think you agree with them, all the while planning your policy on something that a lot of them would not agree with at all. It's hard for me to see it as anything else but dishonesty of exactly the type that Senator Obama consistently says he's going to be a change away from. (He surely would be a change from the current administration in terms of policy, but that's not really the thrust of his change rhetoric.)
Update: Obama's response to Hillary Clinton's changing positions on Iraq is a good example of his condemnation of the very thing he does on this issue:
I have to say that she started this campaign saying that she wanted to make history and lately she has been spending a lot of time rewriting it. I know that in Washington it is acceptable to say or do anything it takes to get elected, but I really don't think that is the kind of politics that is good for our party, and I don't think it is good for our country, and I think that the American people will reject it in this election.
I sure hope the American people will reject it in this election, but it's becoming clear that rejecting it would mean rejecting Obama's candidacy. McCain's changed some of his views, but at least he's giving reasons for those changes and admitting that he's changed his mind. Obama is completely contradicting his previous statements while pretending he hasn't changed his views at all.