The DMR endorsement is indeed an important page turning moment in Hillary's campaign. But perhaps even more significant is the criticism of Obama today by Paul Krugman. Say what you will about the guy, but when he decides to take a candidate to the woodshed over a particular issue he is often right (remember his frequent attacks on Bush in 2000 over his social security privatization scheme?).
The problem with Krugman's criticism is it's a little complex, and difficult to fit into the bumper sticker slogans of the primaries. People tend to think of Obama as more progressive than Hillary and don't realize that on a number of issues he's in fact articulated a more conservative, almost Lieberman-esque view - the "can't we all just get along argument" which has basically killed the chances of any real progressive agenda in this country.