Recommendation: ClamCase's four $149 models--a black one for the iPad, and black, white, and black-and-white models for the iPad 2--offer a nice combination of clever design, good protection, and a decent (but not great) keyboard. Their best attribute, though, is that you can flip the keyboard around to the back to turn a ClamCase-encased iPad into a bulky tablet. (When doing so, you can put the keyboard to sleep to avoid accidental typing.)
Honorable mentions: Best Buy house-brand Rocketfish's Advanced Series Keyboard Capsule for Apple iPad 2 (iPad 2; $100) has better keys than the ClamCase, but its nearly vertical iPad position when open can make it difficult to see the iPad's screen. The Advanced Series iCapsule Keyboard for Apple iPad (iPad; $100), Rocketfish's version for the original iPad, is much bulkier, but it offers a similarly nice keyboard with an adjustable screen angle, and it's currently available for half price if you can still find it in a Best Buy store or online.
These models integrate a keyboard into a rigid shell that protects the front or back of the iPad in transit. (Most cover the iPad's screen, leaving the backside exposed.) When you're ready to type, you pop the iPad out of the shell, stick it in a prop-up slot above the keyboard, and start typing. Most keyboard shells offer only a single prop-up angle for your iPad, though they often let you use your iPad in your choice of portrait or landscape orientation. As with clamshell-case keyboards, the keys here tend to be small, but decent; the models I've recommended here have very good keys. Keyboard shells can be used on your lap if you're careful, but they tend to be less stable than folios and clamshells.