1 After reading the full account of both interviews you link to, I disagree that McCain has "flip-flopped" regarding the substance of his anti-tax increase statements.

In the Ponnuru interview McCain addresses what he would do "If [he] could get the Democrats to agree, or at least to come to the table" on the matter of tax simplification; however, in the Stephanopoulos interview, he explains how he would get that to happen. He says to Stephanopoulos, "No, I have said and will say, I will say that everything has to be on the table, if we're going to reach a bipartisan agreement. I've been in bipartisan negotiations before. I know how you reach a conclusion. We all have to sit down together with everything on the table."

Compare this to what he says to Ponnuru about a previous issue, "And the second thing I would’ve done, I would have made it very clear to the American people that the Democrats would not sit down and negotiate. In 1984 for better or for worse Ronald Reagan and Tip O’Neill stood together in the Rose Garden. You know, I didn’t take any kind of a leading role in this issue, I really didn’t. I’m all for reform, but I just wasn’t. Lindsey Graham did. Lindsey Graham sat down with a bunch of Democrats, individually or one or two. And said “Let’s try to work this out,” and he just got no response. You see what I mean?"

Based on this further context, it seems clear to me that there are two distinct issues: A) McCain's view on tax increases and B) McCain's strategy for getting Democrats to negotiate on tax/entitlement simplification. It seems to me that in the Stephanopoulos interview McCain has NOT flip-flopped on his views ("I don't want tax increases."), but rather is outlining his strategy to get the Dems to sit down and actually get something done for tax reform.

I suppose you could say that this strategy is very different from that one that he initially told Ponnuru ("I would have made it very clear to the American people that the Democrats would not sit down and negotiate."), but I'm willing to allow McCain the benefit of the doubt b/c he goes on to criticize the media opposition to Bush's attempts at reform ("You go to the American people, you go on television and you go out on the hustings: all of those things that the President did. But I don’t think that they got the message of how broken the system is. [...]All of the media coverage seemed to center around retirement savings accounts[...], but somehow the media [made it]: “Bush hypes retirement saving accounts.” I would’ve liked to have seen the headline: “Bush: System is going to go bankrupt. Present-day workers will not receive the same benefits as present-day retirees.”).

I'll bet McCain has changed strategies b/c he now knows that the media will not treat him as their Maverick darling and he won't get the exposure necessary to exercise public leverage against the Democrats' stonewalling. So, he's switched strategies and has to bring them to the table somehow in order to get something done. But that doesn't mean he's "flip-flopped".