I'm on a conference call with Clinton operative Harold Ickes, in which he's floating a new-ish argument about why Hillary would be the stronger Democratic nominee (this was in the context of the decision facing superdelegates): Hillary has won key general-election swing states like Nevada, New Mexico, Arizona, and Arkansas, while Obama's won a lot of states Democrats will have zero chance of carrying in November, like Nebraska, Kansas, and Idaho.
Hmmm. If we're now talking about potential general-election swing states, it seems pretty clear that Obama's won as many as, if not more than, Hillary: Colorado, Virginia, Minnesota, Iowa, and Missouri all come to mind. Moreover, with the exception of Missouri, Obama's winning these states by large--in many cases overwhelming--margins. Finally, does anyone really think Arizona's going to be a swing state in a race involving John McCain? This seems hard to believe.
Update: Brendan Nyhan looks at the ten closest states from the general in 2004 and notes that Obama has won six of them in the primaries. (As he notes, you obviously have to be careful about extrapolating from the primaries to the general. The point is mainly to assess the Clinton campaign's logic on its own terms.)