Jose Reyes had a career .338 OBP in 5 minor league seasons. This season he has a .399 OBP, easily a career high (last year he had a GREAT season, with a .354 OBP). In all those minor league seasons he never ONCE had a season or stint with a minor league team with an OBP above .359 (as a 17 year old in the rookie-league).
While "struggling" at AAA, Ellsbury has a .362 OBP; and between AA and AAA he has a .408 OBP.
I think people here are knowingly being conservative about his speed. This kid has amazing speed. Not 40 SB territory, but 60-70 SB territory.
In 162 games, at the rate he stole bases in A, AA, and AAA, Ellsbury would have had 62, 84 and 73 stolen-bases respectively. The catchers will get better, but not THAT much better; especially given that Ellsbury is bound to become a better baserunner with experience.
I leadoff with a guy like that. The desire to put youkilis at the top even if Ellsbury has a .380 OBP seems like a cute sabermetric move, but I don't think the Sox would do it.