Congress could change the date of elections, I beleive, but it's questionable whether they could do so in the midst of a campaign. Along the same lines as Bush v. Gore, they might not be able to change the rules in the middle of an election because of the vesting of a right to vote, etc.
Also, the rules of the DNC (and RNC) for that matter say that the National Cmte can either recall the convention or chose itself a new candidate in the event of a death. However, I'm not sure how this rule stacks up against state law concerning whose name appears on the ballot. The Dems could do so anyway, because which ever electors were chosen could vote for the new nominee/candidate.
Personally, I think the more chaotic eventuality is if Obama manages before the convention to amass a definite majority of delegates (probably unlikely as of now) and then is assassinated. Same goes, by the way, for John McCain being shot just before the RNC.