This is a somewhat strange move, given that the Cubs could have potentially saved a truckload of money by waiting just a few weeks to bring him up. And with Theriot playing so well (leading the league in hits), it's also an odd time to move him to second base.
It sounds like Chad Tracy will be moved to the minors to make room for Castro on the roster.
I'm not entirely sure what to make of this yet, but the positive view is that it's pretty exciting to finally have the Cubs' hottest prospect joining the big club. After a miserable set against the Pirates, it's clear the Cubs could use a boost. While it's unfair to expect phenomenal production from Kid Castro, perhaps he can inject some life into the team.