According to a colleague of mine who works with AT&T (who has been fairly accurate in the past), Apple and AT&T have been developing an iPhone tethering plan, similar to that allowed for Blackberry users. The reason we haven't heard or seen anything about it, is because AT&T is being extremely paranoid of how the service will run on their network. "Since I've worked here, I've never seen them be so worried about legalities," my colleague explained. Regarding the service being available to Blackberry users, he says that it's a different situation. Blackberry users haven't slammed AT&T's network by the millions in one quarter, in such large rushes. "Regardless of how many billions of dollars AT&T pours into their 3G network, it hasn't been stable enough to handle all you iPhone users," he explained.
MacBlogz has confirmed that Apple and AT&T are indeed working on it but it has been delayed due to lack of confidence in the network, should a large percentage of iPhone users begin using the service.