I think that would be a mistake and this election season has shown why. You would be looking at a national result like Iowa's. I want a candidate that can weather a few storms, not just be blessed with sun on one day.
I'd rather see five primary dates four weeks apart. Divide the states into five groups of roughly the same number of delegates. Rotate the order every four years.
Romney and Santorum won because neither has been trashed by the media. Santorum had little support on Free Republic until a week or so ago, which means just like the voters in Iowa FReepers will panic and run from candidate to candidate based on media coverage.
The anti-Santorum articles have already started. Unless most of the others drop out there's a great chance he'll be back in the pack in a couple of weeks, much like happened to Bachmann, Perry, Cain and Gingrich. If we had one primary day we'd be stuck with Romney.