(Updated April 26, 2012, with two new keyboard cases.)
The one differentiating feature separating the iPad from portable computers such as Netbooks for many people, other than the operating system, is a keyboard. Yes, the iPad has its own onscreen virtual keyboard, but some will be tempted to explore alternatives to get "serious" writing done.
Keyboards for iPads have drawbacks: they add bulk to your bag, and the Bluetooth connection will add some drain to an iPad's battery life. You'll also have to consider whether you prefer a standalone keyboard, or a case that has a keyboard built in. Some cases have compressed keyboards, and others manage to fit a full-size one. You'll also have to learn to live without a trackpad.
For your comparison-shopping help, here are the keyboard cases I've reviewed.They're categorized by iPad: the first few fit the third-gen iPad and iPad 2; the ones below that are largely iPad 2 specific, but a few also fit the original iPad. Some may also fit the third-gen iPad, but we haven't gone back and tested them on a case-by-case basis (if it's a sleeve-type case, it could fit; the snap-back attachments most likely won't.)
I have a new favorite keyboard case, although it's more of a magnetic cover accessory: the Logitech Ultrathin Keyboard Cover. For a folio-type case, I'd recommend looking at the less exciting but effective Belkin YourType Folio + Keyboard. And, if you're looking for a standalone keyboard with a built-in stand, consider the Logitech Tablet Keyboard. Do you have a favorite? Sound off below.
Third-gen iPad and iPad 2-friendly:
Logitech Ultrathin Keyboard Cover for iPad
Logitech's Ultrathin Keyboard Cover might be the best keyboard accessory ever made for the iPad, if you're looking for a highly portable and functional keyboard that travels light; just make sure you carry a separate protective case. Read the review.
Belkin YourType Folio + Keyboard for new iPad and iPad 2
Belkin's iPad 2- and third-gen-iPad-friendly YourType Folio + Keyboard case takes a solid if uninspired folio case design and adds a keyboard for one of the better protective solutions around. Read the review.
Logitech Fold-Up Keyboard for iPad 2
Logitech's latest iPad keyboard product is mechanically ingenious: a push of button and the case unfolds to become a Bluetooth keyboard and a stand for the iPad 2. Pull the keyboard halves apart, and the fold-up keyboard collapses, turning into a thick but functional iPad 2 case. You can also attach a Smart Cover to the front to protect the screen.
There's one drawback to this fold-up marvel: the keyboard just isn't as good as some other Logitech keyboards we've seen, exhibiting a bit too much flex. Still, office-gear show-offs might find this to be the best desk solution yet.
Targus Versavu Keyboard and Case for iPad 2
Targus' previous Versavu case was notable for its solid construction and capability to swivel the iPad 2 around into landscape or portrait orientation. The keyboard-added version of the Versavu has a thick, protective leather outside and a Bluetooth keyboard ensconced in padded microfiber.
It's the best case we've seen that has a keyboard, but the Versavu works better when the iPad 2's in vertical portrait mode, and the keyboard compresses some of its punctuation keys in an attempt to add space.
Zaggfolio for iPad 2
Zagg makes excellent iPad keyboards (see below), but its iPad folio-style keyboard case is a mixed bag. When folded down into keyboard mode, it's excellent. However, the flimsy-feeling plastic shell pales in comparison with more form-fitting alternatives, and it's hard to remove the iPad 2 once it's in the case.
Interesting plus: the keyboard can be removed from the case, and the iPad 2 will still stand up in the standalone keyboard. Why, then, do you need the case?
Logitech Keyboard Case by Zagg for iPad 2
Zagg's slim, aluminum keyboard doubles as a case of sorts for the iPad 2 (there's also a version for the original iPad), although it only covers the front face and leaves the back awkwardly exposed. Some people might get freaked out by pressing the iPad 2's glass against the keyboard, but rubber bumper pads keep the two separated.
Our proposal: don't consider it a case; think of it instead as a superportable Bluetooth keyboard that has its own stand and forms an attractive and functional package when placed on a desk. For commuting distances longer than the local coffee shop, you'll still want to put this keyboard/iPad combo in a sleeve.
Standalone keyboard alternatives that aren't cases, but work with any iPad:
Logitech Tablet Keyboard for iPad
Instead of a keyboard case, some might prefer going with a Bluetooth keyboard for their iPad instead. One of the best we've seen that's optimized for the iPad is Logitech's. The wide, full-size keyboard (nearly the same as Apple's Bluetooth Keyboard) feels great and comes with iPad-specific hot keys. Typing on it feels as good as typing at a regular desktop computer. And, unlike the keyboard cases we've seen, Logitech's keyboard uses AAA batteries.
Its most impressive feature, however, might be its sleeve: the magnetically hinged cover folds back and becomes an iPad stand that's perfectly angled, and will accept the iPad in landscape or portrait mode. This stand will support the original iPad as well as the iPad 2 and third-gen iPad. Android users, rejoice: Logitech makes a version for you, too.
Apple Wireless Keyboard
This isn't a keyboard case at all, but Apple's $69 classic is the go-to keyboard for many, simply because you might already own one: it comes standard with the iMac. The clean, crisp white keyboard has excellent response, and its dedicated volume-control and play/pause/skip keys actually work with the iPad. Apple's keyboard uses removable batteries, like Logitech's. It's very thin, but it's also wide and flat, and a bit longer than the iPad. It might require a larger bag than your average iPad murse.
Thanks to the Smart Cover's ability to prop up the iPad 2 and third-gen iPad, it's not such a bad option.