I bought the 3G iPad because there are few Wi-Fi hotspots around where we live, but I hesitated to sign up for another data plan with AT&T because then I'd be spending $65/month ($30 for my iPhone and $25 for the iPad). AT&T could offer a 2GB/month plan that was shared between an iPhone and iPad, but you and I both know that is never going to happen. AT&T treats iPhone customers like a farmer treats dairy cows, and if you don't believe me just look at the $20/month charge for tethering that gives you no additional bandwidth.
So rather than continue to pay for voice I didn't use and double pay for data, I decided to go to one device. After a week with the iPad, it was clear which device I was using more often. The only time I reached for the iPhone was when there was no Wi-Fi around.
The transition will be a little bumpy because my 3GS was was still under contract which means my wife now has my old cell phone number. However, since we both use Google Voice, I can just point her Google Voice number to my old cell phone number and point my Google Voice number to my new prepaid Nokia number. Then it's just a matter of making sure that family and friends know to use that number to call us.
We also turned off and blocked text messaging support to both of our phones. We had both started to go over our SMS allotment each month and were wasting a lot of money on messages that could have been sent via another app like Ping, Textie, or just plain email. We can send/receive SMS through Google Voice without any cost. The issue then becomes "how to we get notified of new messages on our iPhones?" There are two options. The first way is to get Google SMS notifications via email, which is easy enough, and it's free. The second way is to use Smartpush, a $3 iPhone app from Syphir, which will selectively notify you of messages sent to your Gmail (or Google Apps) address. I setup a server-side filter to alert me of new SMS in my Google account via push notifications.
You can send SMS messages using the Google Voice website, but if you are going to make frequent use of it, I highly recommend VoiceCentral by Riverturn. VoiceCentral is an HTML 5 app which is downloaded to your iPhone, which means that when you launch it, the app is instantly ready, rather than waiting for the Google Voice web page to render. VoiceCentral will even let you specify that it should always start in the SMS tab, making it even easier to send them.
I thought about using Skype instead of a prepaid phone (one less thing to carry) but I know there will be situations where carrying the iPad would be a bit clumsy and being able to rely on a regular cell phone would be easier.
Will I live to regret this decision? Only time will tell, of course, but I've spent the last week purposefully restricting my iPhone usage to "phone only" and so far, so good.
How much will this save us per month? Probably not as much as you'd think. Canceling the iPhone 4 order saved us $200 + tax. We can't upgrade the 3GS until February 2011 -- by that point we might as well wait for iPhone 5. The biggest savings will be from cutting ourselves off from SMS, which we could have done anyway (just call AT&T and tell them you want to block them). The $25/month I would have paid for iPhone data I'll now be paying for iPad data, although there may be some months that I don't use it. We'll save about $10/month on the voice plan (we'd save more if AT&T offered a package with fewer minutes, but they don't). It remains to be seen how much the prepaid cell will actually cost in practice.
The savings may be modest, but this way we are saving $10-$20 per month instead of increasing our spending by $25/month. Practically speaking, we're saving about $45/month, maybe more.
UPDATE July 24th: I may write more about this, but our first bill after making this change was $157 compared to the previous bill which was $250. Even when you factor in the $25 I spent on iPad data, that's a savings of about $68.