A Clinton/Obama or Obama/Clinton ticket wouldn't make much sense, and most people on both sides of this competition actually wouldn't like to see such a ticket according to polls. I don't think either one of them are interested in becoming the Vice President.

We've seen long primary competitions before, and people haven't had that hard of a time rallying behind the winning candidate.

We're much more likely to see a rather unknown running mate (I believe the Governor of my home state of Ohio, Ted Strickland has been rumored as a possible running mate for Clinton, though I personally seriously doubt it), or perhaps a ticket with Edwards on it.