The numbers at the moment are interesting. If you look at the CNN Delegate Scorecard, as of right now Obama holds a slender lead in terms of the "pledged" delegates -- the ones who were chosen due to the primary process. But Clinton leads overall, because she has promises from more of the "superdelegates" -- the party honchos who get a vote because of their position in the party.
So let's look at a moderately likely scenario. The convention rolls around, and the race is still too close to call. Obama still holds a lead in the pledged delegates, Clinton in the superdelegates. Things begin to solidify towards Clinton *because* of the superdelegates. What happens?
Hard to say -- not riots, because people generally need better reasons to riot nowadays, but massive and vocal unhappiness among the party. The convention itself turns into a huge scandal, as the pundits talk up the anti-democratic nature of the superdelegates. The superdelegates come under *enormous* pressure to swing their votes to match the popular vote, and people start talking loudly about eliminating the superdelegates entirely. And the whole thing does a fair amount of damage to the Democrats, who look chaotic next to the coronation of McCain (with the hardcore conservatives quietly holding their noses) happening over at the Republican convention.
I do hope the party leadership is ready for this, and thinking about how to react, because it looks to me like it has a fair chance of playing out just this way. They will undoubtedly make lots of noise about how the system is so much better than back in the days of backroom deals, but I don't think that the modern electorate is going to have much sympathy for that. The superdelegate system has continued for many years precisely because it hasn't mattered much. If it *does* start to matter, I think it's going to turn into quite the national stink...