Specifically, users who paid to have their 3G iPhones unlocked by Vodafone* are unable to re-connect to the service following the update, and instead receive a "0xE8000001" error message. How helpful. It seems to happen on both Windows and Mac OS X.
As you remember, Vodafone is selling the iPhone in 10 countries, and a quick look at Apple's Discussion Boards reveal this problem popping up in Spain, Italy, Sweden and more, which leads us to believe that it's Vodafone's issue.
We haven't come across a fix yet, but we'll keep you posted. In the meantime, let us know if you've been able to squash this bug.
Thanks to everyone who sent this in!
*Update: Some of our tipsters have had luck restoring the phone to reconnect to Vodafone, but not all. Also, a reader from Finland reports the same issue with his locked phone on the Sonera network, and a Belgian correspondent chimes in with the same problem on Mobistar (Orange); looks like the issue is more widespread than just Vodafone's customers.