IPad Keyboard Dock
Sure, the iPad has its own onscreen keyboard. But the awkward ergonomics--how do you hold the iPad at a good viewing angle and type at the same time?--don't recommend it for serious, long-term use. For that, you're going to need an external keyboard. Apple's own iPad Keyboard Dock would be a great choice.
The iPad Keyboard Dock s essentially Apple's current wireless keyboard with some slightly different keys and a dock attached. But it's more than the sum of those parts.
You can, of course, use it to type in iPad apps. But its dock-connector port and audio output mean you can also charge the iPad, connect external (powered) speakers, and sync with your computer through the Keyboard Dock. With its iPad-specific keys (arrayed along the top, in place of a standard keyboard's function keys), you can control screen brightness and audio volume, perform searches, display photos, and go to the iPad's home screen. (There's also a blank button in the middle of the top row, the purpose of which isn't yet clear.)
The Keyboard Dock feels great to type on; to my fingers, it's indistinguishable from the keyboards on recent MacBooks. Its weight and a non-skid pad on the bottom make it nice and stable; you don't feel like you're going to topple the tablet from its dock if you type too hard or if you have to tap the iPad's screen. And it holds the iPad at a great angle for comfortable viewing.
Several standard Mac shortcuts key combos (including Command-A, Command-Z, Command-C, and Command-V) work in some iPad apps (notably iWork); others (Command-B, Command-I) don't. Additionally, you can insert special characters using the Option key. It's to the Keyboard Dock's credit that I found myself trying to open menus and other bits of Mac-standard keyboard interaction that the iPad doesn't (yet) support. You can't use arrow keys and Return or Space to open apps, and you can't tab between buttons and controls, as you can on a Mac.
My only caveats about the Keyboard Dock are its shape and weight. Because of the dock rising up out of the back, it's not going to slip seamlessly into narrow sleeves. And it's 21.4 ounces, compared to the standard wireless keyboard's 11.4 ounces--not a huge difference, but it's there.
It can also be disconcerting to use it then have to move your hands up to the screen to perform functions that you can't from the keyboard. That back-and-forth can be awkward, particularly if (like me) you're accustomed to using your Mac's keyboard as much as possible.
But those are quibbles, compared to the feeling of typing on a nice keyboard and having your words show up on the iPad screen.
Macworld's buying advice
The iPad Keyboard Dock makes text entry on an iPad feel natural; I could type comfortably enough to write the first draft of this review on it. Using it to type in Pages, I had my first "this isn't just a big iPod Touch" experience; it really could be a legitimate business tool. If you're going to use the iPad for work, you're going to need a keyboard. I can certainly recommend this one.