I'd love to know how Biden was doing at intrade in March 2005 for '08.
Don't know about March, but in July 2005, he was in 3rd place, and at about 10%:
Look at that! I'm part of the historical record!
Generally, Intrade has issues for low probability events. 5% should basically be considered indistinguishable from 0 (though McCain was at 4% or so in Sept. '07, when I came about this close to betting on him...bah.) And if the volumes are as low as I expect them to be here, push that 5% up higher.
Though an Obama renomination can't be considered an absolute lock. Obviously, he could die, in which case Biden would be the odds-on favorite (close enough to '12, you can't very well challenge the guy without it being seen as taking advantage of his death; further away, and Biden has enough time to cement his position). If the economy is still in the toilet as of late 2010 and the midterms went horribly (or there's some other black swan event), I wouldn't count out a challenge entirely. It would have to be someone of Ted Kennedy '80 stature---either Hillary (whose job in State is partially immune from any Obama economic woes, but it would still be awkward unless there was a public, 1915 Bryan style break) or Al Gore (who would have absolutely no desire to run under any circumstances, anyway).
Total odds certainly 3% or below.