Yep, had this problem too. Your carrier info has been hosed. one option, you must reload the ipcc file via an old rev of itunes. Now its been well documented on these forums and others that the newer versions of itunes wont let you do that which is why i say old rev of itunes. for me, i installed virtual box (free VM) on my MB and then installed the (free) windows RC1 and updated it that way. Kinda a PITA, use at your own risk.
Here is the much EASIER fix. THIS MUST BE DONE VIA SAFARI ON YOUR MOBILE!!! so find some wifi to use.
open safari, goto unlockit.co.nz and hit continue, then custom APN and select US-Cingular. You could type in the custom fields if you want, but remember this only gives you the three fields, so this isnt a fix for trying to get tethering or MMS working, but IT DOES fix your cell network issue.
now my next logical question, why the heck can some random site update carrier info via safari and we cant even do that via itunes?
good for submitting this, i did too. there is also an iphone update tool that you can snag on apples website, i dont have the link handy, but it will update the three apn fields as well if the above trick doesnt work.
cheers