Same here, no service after upgrading to iOS 8.0.1. Am in Hong Kong with 3 as my carrier. I tried my SIM card in an iPhone 5 (iOS 8.0) and got service immediately. It is definitely related to this update.
I was the one who posted about a verizon phone with an ATT sim not working, but as part of my troubleshooting I put the verizon SIM that came with the phone back in and I still get no service (and the service is still active on the verizon account), so it looks like it may be more than just an ATT issue.