Update: It's been yanked.
If you follow a rather rococo set of network config steps (detailed by 9to5Mac), make a recording, and rename it "PearSauce269.aac" (really? OK, sure, why not) then voila! Free tethering, regardless of your iPhone data plan (but that will still use up your data allocation). [Update: Note that this will only work from your Mac, not your iPad, as it requires you to create an ad-hoc network to start with. If you went ahead and spent $2 thinking this would tether your iPad, sorry about that.]
Given that the discovery of this feature was coincidentally
and/or conveniently timed for a Friday evening, when the App Store minders have presumably fixie-biked off into the Cupertino sunset, there's at least some chance the app will still be on the store in the morning -- but needless to say this is not a good long-term plan for tethering.
[hat tip Giz]