Last week it was revealed that AT&T is planning to unveil an official tethering plan for the iPhone stating that it was coming “soon”. MacBlogz is now adding further information about what to expect from AT&T:

• 5GB Data Cap (just like Blackberry users) – AT&T will automatically turn off your tethering connection if you use too much bandwidth. Of note, the 5GB cap might get sliced for all users, not just iPhone users, in order to accommodate all new tethering plan customers (bandwidth demand).
• Initial connection through iTunes? (not sure if this is every time you connect, or just for initial setup)
• Expected speeds: GPRS: 30k – dialup speeds / EDGE: 110k – ISDN speeds / 3G: 1000k – slow broadband.
• You’ll want to disable auto-updates for everything, so as to not go over usage limitations.
• Will be +$30/month, new iPhone plans may be rolled out with tethering rolled in.
• Do not pro-rate tethering/bandwidth, it can become a nightmare, instead wait until new billing cycle.
• For unlimited bandwidth, AT&T will tell you to get a wireless PC card.
• No idea on a launch date.