Logic would dictate that since Lance has a proven advanced cardiovascular fitness level (maybe the greatest ever recorded)and muscular endurance level and had done some competitive running before (albiet not recently) that chances are good that if he was willing to seriously train for a significant period of time ( I have my doubts as to his desire to do this) he stands a decent chance of being a pretty good runner.
In my opinion this definately means sub 2:30 and if he is serious probably sub 2:20 but how much under 2:20 depends on several factors that none of us could know at this point and even Lance probably doesn't know.
I'd like to see him do it. I'll be the first to cogratulate him if he is standing beside me on the starting line of the 2008 Marathon trials. I'd have to kick his butt in the race but I'd congratulate him on getting there.