Let me say that I'm not shocked that Senator Obama advisors would tell him that once the nomination is "sewn up" then it is time to shift to "the center".  However, I question that strategy because what exactly is "the center" of the political spectrum? 

Polling by Pew has shown a consistent shift of people who self identify with a political party to skew towards the Democratic Party label.  PEW published this on March 20, 2008:

In 5,566 interviews with registered voters conducted by the Pew Research Center for the People & the Press during the first two months of 2008, 36% identify themselves as Democrats, and just 27% as Republicans.

Rasmussen Reports show that Democrats are more trusted on all ten issues that are polled.  From their report, June 21, 2008:

This month, voters trust Democrats more than Republicans on all ten key issues tracked by Rasmussen Reports. The two parties are almost even on two issues, taxes and national security.

Robert Reich wrote in 2001 about "the center": 

The political "center" is imaginary, and its recent elevation as a desirable place for politicians to inhabit is dangerously misleading. What's more, the politician who seeks to move there is abdicating any semblance of political leadership...

One cannot lead from the center because most voters are already there, and it is no great accomplishment to lead people to where they already are. That's much more like pandering. In this sense, moving to the center implies a politics responsive to the immediate and unreflective desires of constituents, especially those most likely to vote. That's familiar politics to the legions of Washington strategists and pollsters who make their livings charting and responding to such desires. But it's hardly how politics should be practiced in a deliberative democracy.

If the Wall Street Journal is applauding the move to "the political center" then is are the most recent choices that Senator Obama is making the one which will empower people who believe that change for the liberal and progressive movement which America stands: his support for HR 6304 (to amend FISA and allow unlimited spying on Americans just on the word of a man) and his condemnation of the recent Supreme Court ruling to deny extending the death penalty to adults who rape children.

I will point to a Buzzflash.com editorial from January 29, 2006 which points out that change means just that change.  It is not "running to the center" but:

America is always a country that has prided itself on evolving, growing, changing. How could there be a permanent center anyway?

That's the purpose of democracy. Through great public debate we develop governmental policy for the common good of the nation -- and it morphs over time.

To abandon that principle to a myth is a cowardly thing to do.