First off, if you were thinking of buying a Kindle DX, I'd say forget it. The iPad is a knockout punch to that device. At just ten dollars cheaper than the low end iPad, there just is no contest. The two devices are the same size (both are 9.7"), but the iPad has a color screen and can do a lot of things that Kindle just can't do.
Yes, the DX has free 3G wireless for buying books, but the iPad has 802.11n, and for most common usage, it just isn't that hard to find a place to hook up. Book prices may be a bit higher on the iPad, but they will be in color and multimedia with the promise of moving video, color charts and pictures, and so on. I also consider the on-screen keyboard an advantage for the iPad -- the keyboard on the Kindle is basically a waste of space.
As a travel companion, all you can do on the DX is read, although there is a rudimentary browser, and more functionality coming. Subscriptions are another Kindle selling point, but it's not a stretch to believe that Apple has something like that coming as well. Comparing the iPad to the Kindle DX, I just don't think there is any contest, and Amazon is going to be forced to think about how they are pricing and marketing the Kindle DX.