Dont count on saving that much. Currently, AT&T charges PDA and BB phone users an extra $45/month on top of voice and data charges to tether. There's no way they dont do the same with the iphone.
+1 I don't think AT&T is going to let anybody get away with free tethering. Maybe a less expensive tethering plan will do, Verizon only charges $15/mo. for tethering a BB.
On a side note, copy and paste would be nice but MMS would be gravy.