The problem is, you need three to broker a convention. That means Huckabee and/or Ron Paul need to siphon off as many delegates as possible between now and the convention for there to be any chance for a dark horse to emerge, someone hand picked to be well liked among the base but still electable to the masses.
Remember, the party establishment is still pretty much run by the Bush administration by virtue that Bush is still in office, and McCain and Bush still don’t like each other all that much (so much to the point that McCain almost ran as a Democrat VP candidate... wisely he chose against it). Don’t automatically assume McCain’s the nominee if the convention goes brokered, as it possibly will.
The Democrats, for that reason alone, will not have a brokered convention, because only Obama and Clinton have a chance. The more likely scenario is that Obama may win the primaries but if Clinton hauls in superdelegates and, on top of that, gets the Michigan and Florida uncontested election results counted so that she walks away with the Democratic nomination, then all hell will break loose in that party.
That's basically the scenario that I see has a good chance of unfolding. Obama will win a small majority of delegates in the primaries and then you will have the Clinton camp howling that it is unfair to not have the Michigan and Florida delegates counted (states in which the Clintons won handily) and as you state above, all hell will break loose.
We should all remember how the Clintons would not take their name off the ballot in Michigan while most other candidates did (at the request of their party), guaranteeing them an easy win as Obama did not contest there. Then in Florida, the Clintons showed up for "fundraising" right before the primary and had the audacity to claim they weren't really campaigning there. The Clintons knew they needed to "win" those states as insurance, in case Obama happens to squeak out a slim majority of open delegates.
I guarantee you all that if Obama goes into the convention with small lead in delegates, the Clintons will find a way to make Florida and Michigan count after all.