According to a confirmed from iClaridied being given from MuscleNerd that most devices can be downgraded from
The only exception is the 2 which cannot be downgraded at all until a bootrom-level or iboot-level exploit is developed.
This information may be helpful to developers who are thinking about upgrading their device but may need to revert back to iOS 4.x in the future.
For anyone else who is thinking of upgrading to the beta, make sure you remember that your baseband will be updated, affecting your ability to . Downgrading your firmware will not downgrade your baseband. So update with caution.