My report: yes, you can. On a recent trip, I left the MacBook at home and brought my iPad. This also allowed me to leave the big Timbuk2 Classic Laptop Messenger Bag behind; I carried the iPad onboard in an old Eddie Bauer field bag. The iPad, in a cheap $5 neoprene sleeve we got until a case we really want appears, fit easily in the field bag’s main compartment. One bonus is that TSA doesn’t require you to take iPads out for screening, unlike laptops.
For casual writing (emails, mainly), the iPad’s on-screen keyboard is sufficient. When I had to work on a chapter of the latest book, I turned to one app and an Apple Wireless Keyboard (I bought this while on the trip from the local Apple Store). The keyboard is full size, but the function keys are smaller than usual (no number pad, though), very thin and light, but still sturdy, since it’s made mainly of aluminum. It connects to the iPad via Bluetooth. On the way home, because it was a short flight and I didn’t need to write, I packed the keyboard into my roll aboard luggage.
I just weighed my field bag with the iPad in its sleeve and the Apple Wireless Keyboard; it came to 3.5 pounds, still a pound less than my 13″ MacBook alone. When I’m traveling, I’d need the iPad charger and cable (which would go in luggage), and the rest of the stuff I travel with (reading glasses, business cards, earbuds, etc.) would probably raise the total weight to close to 5 pounds.
The app I used for writing was Dataviz’s Documents to Go Premium, which creates and edits Microsoft Office documents (Word, Excel, PowerPoint). The great thing about the Premium version (and justifies the extra $5 over the basic version) is that it connects with the Dropbox online storage service, which automatically syncs any document between your different devices. So I wrote the chapter in Documents to Go, saved it in the iPad and also to Dropbox (you need an Internet connection, of course), and the chapter was immediately replicated on the Mac at home. When I got home, I opened the document in Word from the Dropbox folder on my Mac Pro, did some touchups, and sent it off to my editor. I could have emailed the chapter from the road, of course, but the deadline wasn’t that pressing.
The other benefit of using Dropbox is automatic backup of my work. If my iPad had been broken or stolen, my work would still be safe, both at home, or, if my home computer wasn’t running, on the Dropbox Web site. DTG Premium also supports Google Docs, Box.net, SugarSync, and MobileMe for online storage.
I’ve even experimented with using the free Dragon Dictate for iPad for speech recognition (I’ve used Dragon Naturally Speaking on the PC for more than a decade). I think it’s OK for things like emails, but it’s not ready for serious work. It requires a fast net connection to work at all (the recognition is done on their servers), it’s not good for a lot of text at a time, and you can’t train it to the idiosyncrasies of your voice.
In any event, I’m sold on the usefulness of the iPad as a laptop replacement for writing. For quick to medium-length trips (up to about two weeks, for me), I think I wouldn’t miss the laptop at all. If I had to do a presentation, I’d still bring the MacBook, because Keynote for Mac is markedly better than Keynote for iPad (the book I’m doing now is about the latter, so I’ve become intimately familiar with both versions).