Fantastic news via Marc Lancaster's twitter update.
B.J. Upton will assume the leadoff spot once he returns to the Rays lineup. Give the most at-bats to one of your best on-base guys? Dusty Baker hates this move. In the interim, Akinori Iwamura and Jason Bartlett will "platoon" in the spot. Iwamura will leadoff vs. right handed pitchers with Bartlett handling the left handers. Once Upton is healthy, the double play combo will fill out the eight and nine spots in the lineup.
Bartlett's career line versus is lefties far surpasses his one versus righties. We're talking nearly .200 OPS points. Iwamura hits basically the same versus either hand - in fact, he's been better versus lefties - but if we're talking production possibilities, it makes sense for Iwamura to hit leadoff against righties more than it does Bartlett to hit leadoff against righties and vice versa.
The benefits of this are going to be short in duration. During that week, we're looking at least one lefty in Boston (Jon Lester) and one in Baltimore (Mark Hendrickson), so Bartlett's leading off for two games and maybe never again.