Users are experiencing problems today updating to iOS 5 on their iDevices. The error comes after the download is complete and the iDevice is ready for restore. A generic error pops up which just says that the update failed it does not really provide any useful information to the user.
The error is shown above, this error is likely because the Apple servers are being over loaded with users updating their phones at the moment. If you wait a couple hours and then try to update your iDevice again to iOS 5 it should go through just fine. Apple sends you to this relatively useless page.
I however cannot wait that long and will keep trying to update my phone till my fingers bleed or my head explodes. Happy updating!