MacBlogz has a source who has shed some light on a few specific pieces of information pertaining to iPhone tethering we can expect from AT&T, when the plan is finally released.

Details include:
• 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).
• Expected speeds: GPRS: 30k – dialup speeds / EDGE: 110k – ISDN speeds / 3G: 1000k – slow broadband.
• Will be +$30/month, new iPhone plans may be rolled out with tethering rolled in.

More details in the full article here.