Sup fellas, i have an iPhone 4 on 5.0.1 i wanted to restore it but it wont restore at all, not even with through the legit update from itunes, it gives error 2100 and sometimes when trying custom firmware with the phone on dfu mode gives error 1600, Took it to ATT to activate it and the damn thing wont even get service and is an ATT phone, was activate it with it before. Any ideas?

I'm having the similar problem. Trying to upgrade from 4.2.1 to 5.0.1 but kept getting 3194 or 2005!!

Apple no longer signs for 5.0.1

as aneagle rightly points out, Apple no longer signs for it so it is not possible to revert to 5.0.1. 5.1 is your only option unless you saved your SHSH blobs from your 5.0.1 installation. You can then use iFaith to restore it but only on A4 devices.