I've seen a variety of attempts at defining triangulation over the last few days; Dinauer's is my favorite by far. What is triangulation, really? I'll tell you, and you'll enjoy it, but first I'll make you sit through a couple of paragraphs about how bills pass in different contexts.
With unified government, the best course for a president is usually to pass legislation by mobilizing his party. That's pretty much what Barack Obama did during the 111th Congress. The trick is going to be, always, to keep the handful at the extreme left (for a Democrat) happy while also appealing to the 218th most liberal Member of the House and the 60th most liberal Senator. Barack Obama may have, in some sense, wanted to be bipartisan or postpartisan or whatever, but the easiest coalition for almost everything he wanted to get done was going to be highly partisan.
When there's divided government, the calculus changes. While it's still possible that there will be issues in which the easiest winning coalition is constructed beginning with the left and moving to the center, there are other potential available coalitions that involve finding things that both sides really want that the other side doesn't mind that much. That's obviously the case with the tax cut deal: liberals don't care nearly as much about tax rates for the rich as do conservatives (yes, they care a lot -- but not nearly as much). Conservatives do not, it seems likely, oppose UI extension nearly as much as liberals favor it. What this all boils down to is that in the next Congress, there are going to be things that pass with the support of both John Boehner and Barack Obama, and perhaps without the support of some Democrats. Or else, nothing is going to pass at all.
Now, what's "triangulation" in that context? Nothing. Triangulation is an advertising slogan coined by Dick Morris to advertise himself -- to give him as large a share of the credit for Bill Clinton's 1996 re-election as possible. That's all. Trying to find any deeper meaning in it is like trying to find the deeper meaning in "Coke Adds Life" or "Tiger in Your Tank." Might be interesting to do it, but it's not going to tell you much about soft drinks, gasoline, or politics.
That's what those sort of people—Dick Morris, Karl Rove, James Carville—do; they make up fancy slogans or theories or whatever as a way of claiming that their mysterious voodoo is irreplaceable. Now, some are good at what they do, and some are not so good, but the truth is they are for the most part interchangeable; it's mostly luck about who happens to be in the right chair when the music stops. That is, as much as I do happen to enjoy listening to Carville talk, if his presidential race had been Dukakis '88 or Mondale '84 instead of Clinton '92 (or even if it had been Harkin '92 or Kerrey '92), well, no one would think that crazy guy who looks and talks funny was a genius. And Carville, I suspect, was awful good at what he did. Dick Morris? Has Dick Morris ever said anything that made you believe he was good at anything other than self-promotion?
So: liberals were frustrated in 2009-2010 because Barack Obama had to get Ben Nelson on board, and they're going to be furious sometimes in 2011-2012 because Obama is going to find things that get John Boehner on board. Liberals, of course, should do what they can to fight for what they want, and make sure that deals that are made are the best deals they can get. But as for trying to figure out what triangulation is and whether Obama is doing it...well, you're better off trying to figure out whether Danica Patrick is really that into Go Daddy.