While it's way too early to have a guess on where his BA will end up down the road, he's got a strong track record and came to the majors with a reputation for being able to make adjustments.
While his BA remained in the .260-.280 range in his last 3 stops in the minors, he was young for each level, reaching AAA at age 22. The thing that really stands out is the improvement he made each year in his K and BB rates.
2004, age 20, A - 145:66 K:BB ratio in 465 ABs
2005, age 21, AA - 129:70 in 466 ABs
2006, age 22, AAA - 71:52 in 402 ABs
While that success hasn't yet carried into the majors, he's only 23 and he's been a .266-320-.512 hitter out of the leadoff spot this year. I don't think the kid is peaking at age 23, and I think he's the type of talent that'll take this season and learn from it in a big way. His history says he will.
In July and August, he's walked once every 10.8 ABs (once every 20.1 ABs for the first 3 months). He's also been striking out more, hitting more HRs, and stealing more bases. I guess you win some and lose some. He's getting more comfortable, and when his BABIP comes up a bit (it's only about .250 right now), he's got a chance to be a true 5 category stud.