First, my main source for this is a great piece by Rhodes Cook on Larry Sabito's Crystal Ball '08 http://www.centerforpolitics.org/...
Anyway, without getting too deep into the methodology, it's interesting to note that in the two Democratic primary seasons where the popular vote was razor close, we got clobbered in November. I'm not saying that will happen in 2008, but it is a historical fact. On questions like this the "sample size" of history is just too small to mean much.
In 1984, Walter Mondale beat Gary Hart by a total of 307,246 votes in the primaries. Jesse Jackson was also a major factor in that race.
In 1972, Hubert Humphrey actually beat nominee George McGovern in the popular vote by 67,981 votes. Several other candidates were factors too. By almost any standard, 1972 was closer than 2008, no matter how much one wants to jiggle the figures depending on questions such as what to do with caucus states, Puerto Rico and the "flawed" (to put it mildly) 2008 Michigan primary. It is important to note that although McGovern won the nomination in 1972 he really did not have working control of the Convention -- that hurt.
1980 is often cited as a close election in the primaries. Ted Kennedy went all the way to the Convention but the popular vote was won by Carter by a wide margin. Of course, for an incumbent President Carter had a weak showing and, of course, was defeated by Reagan in a landslide.
By any fair reckoning the caucus states should count. My opinion is that those states should be weighted according to thier importance in the general election, and by that standard Obama would win the "popular" vote by a comfortable margin. In an election this close, as shown by RenaRF's EVCELLENT front page diary, counting the popular vote is a nuanced, tough question that is depermined as much by assumptions as math.
The solution: abolish caucuses, open the primaries, count ALL the votes equally and downgrade the Conventions to a ceremonial gathering. For Example, have the delegates simply report the votes -- "Mr Chairman, the great state of XXXXXX, home of the championship college billards team and blah, blah....voted xxx,xxx for candidate x and xxx,xxx for candidate y. I think it could happen. Conventions are great fun, I've been to five, but they are obselete relics of the 19th and early 20th century. The only reason they still exist is because of TV. As the age of TV dims, it's time to turn the lights out at the conventions too.