Problems when downgrading from 1.1.3 to 1.1.1

[dancinfoo1914]

I bought an iPhone around November. I believe it came with 1.1.2 on it. When 1.1.3 came out, I was one of the first to upgrade it. Then I found out about jailbreaking and such, I decided that I wanted to do that. But all the methods described for 1.1.3 iPhones meant already having a jailbroken 1.1.2 or 1.1.1 iPhone. So I tried to downgrade mine to a 1.1.1. It worked well until the very end when the home screen came on and said that I had an Incorrect SIM. I've never taken it out before, and I've used AT&T for a while now. ANyone know what's wrong or if I can fix it?