Appmodo reports that the iPhone MMS is coming in July, and that tethering will be $55 a month. This can't be true: tethering simply means that AT&T is allowing the iPhone to share its internet connection. To nearly double the data plan's price as a result means either that AT&T's network simply can't handle it, and it's pricing it to kill it, or it's just concluded that iPhone users are gullible spendthifts.
Tethering is a $10-$15 extra on other phones: even that is a scam, given that you've already paid for the bandwidth. One does not require a tethering plan at all on Windows Mobile handsets, in fact, which are easy to dupe into sharing their connection using PDANet.
Prediction: this is a seeded rumor, designed to make the eventual $25 tethering plan appear generous and cheap.
Update: AT&T says that it won't be $55 on top of data.