Rachel Weiner over at WaPo wrote a good piece about what would happen should we ever reach a point in which no candidate has half the delegates after the last primary or caucus; she talked to me and to Josh Putnam, and I think she did a good job. I'd emphasize, however, that there's considerable uncertainty about what would actually happen if we really had an unknown nominee when the convention opened.
For example, consider this comment from Solracer to a previous item of mine about deadlocked conventions:
I was a Dean delegate at the 2004 DNC and in that case Howard Dean asked us to vote for Kerry in a sign of unity which most all of us did. So there is still an ability for candidates to deliver their delegates. The big wild card this time however is time itself. In the old days the delegates were party insiders who could afford to take weeks to come to a conclusion. Today the delegates are likely common people with real jobs who are paying their own hotel bills and who have non-refundable tickets to fly home. So either everyone will vote for a single candidate at the last minute just to be able to go home or even worse they will leave anyway and there won't be enough delegates left to put anyone over the top. IMHO the latter is the real disaster scenario and I really don't know what could be done in that situation.I'm certainly not a historian of 19th century conventions, but my memory is that this was in fact an issue at times: who is actually in the room may well have a major effect on who wins. Whatever is true about then, it's an excellent point about what would happen now. Of course, it's quite possible that things would get resolved before the delegates actually met. After all, communications are somewhat more reliable now than when the Whigs were doing the nominating.
Just to remind everyone: if we really got as far as the first day of the convention without resolving the nomination, there's a good chance for major chaos. Everything, from which delegates are seated to the procedures under which the nomination is considered, would be up for grabs. Delegates, and party factions generally, are unlikely to trust anyone to be a neutral referee. Party politicians and others with a major stake in the November election results will have a powerful incentive to keep things looking happy, but the candidates and many party factions may not. Of course, all of this is one reason why the system has been designed to reach a resolution in spring, and I believe that the parties have done a reasonably good job in doing so. But if it every fails...well, it certainly would be fun for political junkies, but it could easily get very ugly for the party.