The plan will cost at least $30 a month, and the cost will be rolled into a new plan for the iPhone. The source said that the plan will cap data transfer at 5GB, and AT&T will terminate the tethering connection if you go over the limit.
Also mentioned is the connection process: iTunes may be required to at least set up the initial tethering configuration, and may be required every time you connect.
Speeds will apparently start at 30kbps for areas serviced only by GPRS, and top out at 1000kbps for a really spectacular 3G connection. No release date was mentioned.
All this comes from deepest Rumorland, of course, so all due caution is advised. MacBlogz says the source has worked at AT&T for "some time now" and the rumor could be taken "relatively seriously."