Apple released iOS 5 last month and there is still no way to downgrade iOS 5 other than the latest. In fact if you have iOS 5.0 and want to restore your device and can not return it clean to let in the same version, to force you to install the latest that is available at this time iOS 5.0.1 Musclenerd just posted on twitter that hackers are working on solving this problem, and got some progress. It seems that Musclenerd and another member of the Dev Team have had some success in downgrading iOS 5 but only for terminals that can be exploited with limera1n.
Cross your fingers and expect some progress in the coming weeks. Solving this problem is more important than launching an untethered jailbreak solution, so it will take several weeks until we have new jailbreak solution available.