Epson has released a slick little iOS app called Epson iProjection (free), which allows iPhones, iPads and iPod touches running iOS 4.2 or later to connect to certain Epson projectors wirelessly.
Epson says the app is ideal for business users who need to travel light yet maintain the capability of presenting with a full-featured projector. The app also should be of use to teachers in the classroom. Epson iProjection is compatible with applications like Microsoft Word, PowerPoint, Excel and Apple's Keynote, plus JPEG and PNG image files.
As for compatible projectors, you'll find a full list here.
One disadvantage of the app is the inability to play either animations or video. It might have been nicer if Epson had simply decided to support AirPlay in their projectors, but that would require additional hardware in the projector. Perhaps it is something we'll see in the future from Epson or others.