I updated my iPhone 3G this morning as soon as the update became available. Everything seemed to go without a hitch and all of my content, apps, and settings appeared to transfer over just fine. However, when I tried to check email, surf the Internet, or perform anything require a data connection I received the following error pop-up, "Could not activate cellular data network: You are not subscribed to a cellular data service." The thing is, I do pay AT&T $30 per month for my wireless data service so I am indeed subscribed to a cellular data service.
I tried checking settings on the iPhone and even verified that the iPhone was the device that appeared in my AT&T account, but nothing seemed to resolve the issue. I then did a bit of Googling and found some methods to try on the macmend.com site. I tried the first solution of resetting my network settings and then after my device reset it took several minutes where the upper left of the iPhone showed Searching... It eventually changed to AT&T with a 3G icon, but still would not connect.
In the past, maybe that was with my older jailbroken iPhone, you could view the Cellular Data Network settings and change the APN, but I no longer see this option on my iPhone and think AT&T blocks this setting access.
I then realized that my plan may not be provisioned now directly for the iPhone since I bought the Nokia E71x a couple of months ago and AT&T may have changed over my data plan (same pricing). On my account page, the iPhone 3G still shows up though so I am not sure if that is the case. As a final test, I tried to perform a full restore of my iPhone to see if that worked, but I kept getting the message that the iPhone activation server is temporarily unavailable.
A call to AT&T is my next course of action, but I decided to give up for now and pop the SIM into my Nokia N97 where data works just fine and I have the control over my access.
UPDATE: I read a buddy's Twitter post that he was having the same issue with me after updating his original iPhone and trying to use his BlackBerry SIM in the iPhone, which worked just fine before the update. Bill Miranda posted a link to the Limited Edition iPhone site where they give you all the details you will need to fix this issue. I recommend just visiting the site they state, via WiFi of course, since that is the quickest way to get back up and running. My iPhone 3G now has the 3.0 firmware on it and everything works just fine with my E71x SIM. I understand this may also solve the issue for those iPhone owners using Pay As You Go services.