According to Business Insider, Sparrow for iOS is approaching its final testing stage:
The Sparrow team is putting the "final touches" on the backend and basic interactions for users. They're trying different "dresses" on the app too, Leca mentioned.
Heavy usage testing will begin in about a week, with tons of fine tuning and meticulous refinements to follow.
A pleasant Gmail experience on iPhone is finally within reach. Sparrow for iPhone will launch within the coming few months.