Aw, pfffft, Hollywood! The movie cast of Rent was pretty long in the tooth, too, but since most of them were the Broadway originals, I think it went over really, really well. Plus they can work miracles with makeup these days! I'd LOVE to see those two reprise their roles. Though now that you mention Reese Witherspoon, I think she'd be an inspired choice for Glinda!

Callie O'Malley is a character on Grey's Anatomy, played by Sara Ramirez. She won a Tony for playing the Lady of the Lake in Spamalot, and I think she'd make a good (if maybe a bit heavy for the role) Elphaba. I can't think of anyone else who'd fit, and can you imagine being painted GREEN every night, and twice on Saturdays and Sundays? Egads!