The iPad Bluetooth keyboards and keyboard cases that exist now are clunky at best, and heavy and unusably bad at worst. The Adonit Writer, however, seems like a pretty quality solution for the problem of typing on an iPad for extended periods of time.
TUAW tested the keyboard + iPad case + outer sleeve and found that not only is it crafted very well from quality material, it's also quite "flexible and secure." The case (not counting the iPad) is 1.1 pounds, so in total, you'll have a carrying weight of about 2.4 pounds—similar to a netbook. TUAW also says the keyboard, which isn't quite full-sized, still feels great.
Adonit Writer was a Kickstarter project that managed to raise $27k (they only aimed at $10k), and will be soon selling iPad 1 models for $100 for the first month, and $110 thereafter. The iPad 2 models are coming soon.
Whether or not an iPad is a better mobile-writing device than a netbook is unclear, especially when you factor in the fact that multitasking on the iPad is clunky needs you to swap back and forth between full-screeened applications. But for those of you who already take your iPad everywhere and want something better for text entry, this seems like a good compromise.