iPad has a pretty large onscreen keyboard. I can type 34 words per minute if iPad is propped up in the $39 Apple case. I can type twice as fast with fewer mistakes on a full-sized physical keyboard. Writing more than a couple paragraphs using the the onscreen keyboard is not only slow, it feels laborious. Apple knows this and sells the $69 Keyboard Dock.
The Keyboard Dock is the same size as Apple's Bluetooth Keyboard. The Bluetooth Keyboard also works with iPad. You'll want some way to prop up iPad since the Bluetooth Keyboard doesn't have a built-in dock. The Keyboard Dock not only props up iPad, it makes an instant connection. There is no setup or software required. Bluetooth keyboards require pairing, and pairing wireless keyboards with class sets of handhelds has had disastrous results in some classrooms.
When a Keyboard Dock or Bluetooth Keyboard is in use, it keeps iPad's onscreen keyboard from appearing. You can see more of your screen with the onscreen keyboard out of the way. That is unless you're using an iPhone/iPod touch app, because non-iPad apps aren't smart enough to hide the keyboard.
As you probably know, iPad cannot display Flash content. That means interactive sites like Glogster, VoiceThread, Myna, and Starfall aren't available on iPad. Furthermore, some sites that do not use Flash do not work properly on iPad. For example, I cannot add, edit, or move notes on a Wallwisher page. Even worse, editors used in blogs and wikis are unusable. While I can view Google Docs, PBworks, Wikispaces, and Blogger pages, I cannot edit them with iPad's Safari browser (this goes for iPhone and iPod touch as well). I have no idea why iPad is not compatible with these editors, but I do hope the situation changes. I would love for me (and any iPad-using students) to be able to contribute to docs, wikis, and blogs. I'd have more of a need for a physical keyboard if I was able to contribute to these sites from iPad.
Seeing iPad standing up behind a keyboard brings back memories of Palm handhelds. Coincidentally, attachable keyboards for those device were also $69. Unlike the Apple keyboard, the keyboards for Palms could fold up, making them easily fit in a container or backpack.
iPod touch and iPhone fit perfectly in the Keyboard Dock, but the keyboard does not work with those devices. While I can type 34 word per minute on iPad, I can only type 18 words per minute on iPod touch. That's where a keyboard would really come in handy! Perhaps iPod touch will work with the Keyboard Dock after Apple releases a new software update.
Buy an iPad and a Keyboard Dock and you're spending at least $568. For that kind of dough, you can get a nice laptop or a couple netbooks...