wait.. are you saying they don't let you do that even out of contract?
The only alternative is to get a laptop data card plan, which IS standalone. But those plans are soft-capped to 5GB a month (which may or may not be a problem for you, who knows?).
So is the iPhone data plan.
Nope, it is unlimited. AT&T makes a distinction between iPhone and Smartphone/MediaNet plans (unlimited), and Laptop cards and phones-with-tethering (5GB cap). But therein lies the gimmick: it's unlimited but only insofar as your phone can "unlimitedly" use data, not an external tethered device.
See for yourself. And actually, it looks like a PDA plan is possible to get without a minute plan, but it's $35. It's also possible to get PDA tethering for $65 (but again, compared to iPhone data + the lowest voice plan, you're saving only $5).
If you are on a data plan that does not include a monthly megabyte allowance and additional data usage rates, the parties agree that AT&T has the right to impose additional charges if you use more than 5 gigabytes in a month. Prior to the imposition of any additional charges, AT&T shall provide you with notice and you shall have the right to terminate your service.