I accidentally updated to iOS 5.1 and now I’m not able to use it. Is there a Gevey Turbo SIM to Unlock iPhone 4 Baseband 4.12.01?
UPDATE: You can now unlock it for free, follow the steps here.
Currently there’s no Gevey SIM or any other type of unlock available for baseband 4.12.01 and 4.11.08.
It’s also not possible to downgrade your baseband by any method. Any website that claims they can downgrade the baseband using software is very likely to be fake.
The only proven method is to replace the baseband chips inside. See here.