They've always been around, but in past elections the 796 Democratic superdelegates have often been an afterthought, an inconsequential byproduct of complex candidate nomination practices and, ultimately, the object of very little media attention.
But unless Barack Obama or Hillary Clinton emerges from the primaries with a clear majority of delegate support (a candidate needs 2,025 votes to seal the victory), the fate of the nomination may very well lie in the hands of the hundreds of party leaders-Democratic members of Congress, governors, additional elected officers, members of the Democratic National Committee, and several former politicians-that vote independently from their constituencies and have until the Convention in August to make up their minds. While Obama's Potomac victories threw him into the driver's seat with regards to "pledged delegates," these folks are the election's true 'x' factor.
Obama is arguing that the "more democratic" option is for superdelegates to vote however their constituencies have voted. That would, of course, also be "better for Obama" because he's leading among pledged delegates. But there are legitimate reasons to keep superdelegates' votes truly independent.