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).