I bought the 3G version of the iPad for a simple reason: Insurance.
It comes down to the fact that the way we use the iPad could be radically different in six months than how we use it that first week. I don't know what developers are going to do with it, what kind of device they're eventually going to mold it into, when it'll be socially acceptable to toss it in my satchel and carry it to brunch, or when maybe—just maybe—I might actually be able to use it to do real work instead of laptop. I don't know. But here's the thing: You can change the software. That's the whole point, even. You can't change the hardware. There is no upgrade.
In fact, there's a hardware reason to buy the 3G model even if you don't think you'll ever tap 3G—thanks, no data contract—it's that only the 3G model has GPS. What do you need GPS in an iPad for? Like with everything else about how I'll be using it in 6 or 9 months, I don't know, yet. But if a killer app comes out requiring GPS that you need to go adventuring with Sir Richard Branson and naked models in the jungle, you won't be able to tag along, because your iPad is gimped.
The math works out, too, because of the incredible deal Apple somehow got AT&T to bend over for. There's no data contract, and you can purchase data as you need it, right from the iPad. I don't plan on actually paying $15 or $30 a month for data all year long. I'm going to buy 250MB of data just when I need it, or if I'm going to rage all over some 3G, I'll pick unlimited data for $30. Then cancel it the next month. No worries, my soul isn't tied to AT&T, life is good. And I'll have 3G data when I need it, not pangs of "crapcrapcrap, I really wish I had 3G right now."
Maybe you think you will add 3G later, and give AT&T the finger in the process by going with Verizon's MiFi. A credit card-sized sliver of engineering wonder, it supplies Wi-Fi to any device, up to five simultaneously, and rides on Verizon's famously more reliable and expansive 3G network. I won't lie, the MiFi's a marvelous gadget, and worth considering if you are looking to pipe 3G to a several devices. But, not only will you still lose out on GPS in the iPad, you'll pay for the privilege of using Verizon's network according, since you'll be paying 60 bucks a month for two years, and for only 5GB of data a month.
I know, my whole argument is essentially based on uncertainty, but the fact is, no one really knows how they're going to use the iPad. A very select number of people on this planet have used it for more than 30 minutes, and the vast majority of software and content, which'll define what it's really capable of, and how we'll really use it, have yet to be created. I'd rather pay $130 to know for sure that mine's able to do everything it could possibly do, and that I'll be able to use it anywhere.
Besides, it'll add to the resale value when I buy a new one next year.
Be sure to see Adam's stingy man argument why you shouldn't get 3G