The only down side is that stand-alone data plans cost a little more. In the case of AT&T, it's only an extra $5 dollars. So if you got a stand alone data plan and 200 SMS messages, it would cost $40 a month, the same as a stand alone voice plan.
If you ever need to make a phone call and you don't have a voice plan, it will cost you $0.40 a minute ($0.69 if you roaming).
"If you have a voice-capable device, unless you request voice blocking, select a data plan that restricts voice access or select a qualified voice plan, the default rate for voice calls on the AT&T's wireless network are 40¢ per minute and 69¢ per minute for domestic roaming voice calls off AT&T's wireless network (rates are subject to change without notice). Additional taxes and surcharges may apply."
What they mean by "Additional taxes and surcharges may apply." I don't know. I'm assuming it the same stuff they apply to everything else.
If you do the math, you'll very quickly find out that if you talk less 99 minutes a month that going ala-carte and not buying the basic $40 a month voice plan will be cheaper. Of course, this is all assuming that you never do any roaming. Plus, you don't get the advantage of having you minutes rollover, obviously.
This is all assuming that AT&T will allow you to buy an iPhone 3G without any voice plan. But I get the feeling for some reason that they'll have some excuse...
EDIT: I think I'm wrong here. If you look on their data plan pages you'll see that they all say this:
"Please note, if you would like to have voice phone service on your device, you must add a voice rate plan to your cart before you add data. Once you have selected a voice rate plan you may select a data service using the Add Services option. Stand-alone data plans do not include voice service."
Talk about a loophole. On one side they're saying that you can go without a voice plan and make calls (at a higher price per minute), then they go on to say that you can't. I guess I just don't know how to read this technical mumbo jumbo.
EDIT 2: My original interpretation of AT&T's mumbo jumbo was right. I dug around on AT&T's support forums and came across a thread that talked about this. This one person has a AT&T Tilt, no voice plan, pays $35 a month for data, and $0.40 a minute for whenever he needs to make a call.
But it's like I said before, if AT&T will let people do this with the iPhone 3G is yet to be seen.
