Microsoft excited a lot of folks with its Surface tablets. The sleek design wrapped around Windows 8 is a big draw, but the keyboard cover seals the deal for many. A similar keyboard cover for the iPad designed by Apple with its characteristic flair would be the biggest selling accessory of all time.
The world is currently waiting for the rumored iPad Mini to be unveiled as I am, but this cover for the bigger iPad would be huge. I predict Apple would sell more of these covers than competitors sell tablets.
I am not an accessory designer nor do I play one on the Internet, but I am confident that Apple could pull this off masterfully. I picture a thin smart cover for the iPad with a sliver of a keyboard similar to the Apple wireless keyboard. The cover would completely protect the iPad front and back, and add a wireless keyboard to the package.
This keyboard would charge with the iPad when plugged in, and would run for weeks if not plugged in for some reason. The cover would support the iPad at a proper angle while used with the keyboard, and also be comfortable used as a pure tablet with the keyboard flipped behind the iPad.
This cover would add little weight and bulk to the svelte iPad, and introduce owners to the utility provided by having a proper keyboard. There are a number of third-party keyboards for the iPad, but none pull off the design and function that Apple can do.
For added benefit Apple could add a port to the cover for connecting a Thunderbolt display to the iPad at the desk. This isn't essential but would open up a world of utility for iPad owners.
I believe Apple would sell millions of these keyboard covers to iPad owners, and would do so as soon as they go on sale. It would quickly become the best-selling accessory of all time for any device. It would also steal some thunder from the Surface tablets with those fancy keyboards.
Update: It seems Apple may be thinking the same thing with this recent patent application.