"Why would you purchase a 17-55 when you already have a 17-70? What would it buy you?"
Constant 2.8 aperture at all focal lengths and from what Ive read, much better image quality and superior build quality. After reading the responses here, though, it doesn't seem worth the price premium. I think I'd be better served purchasing the D7000 and starting a fund for FX lenses for future upgrade.