iOS 4.2 isn't installing on iPhone for some developers if our email box and a quick search of Twitter are any indication.
It sounds like the install starts fine but then has problems during the baseband update and spits out an error code 10. That's hook it up to Xcode and restore to 4.1 language.
That's also why it's a beta for developers, folks, not for production devices.
UPDATE: Justin Horn from WhenWillApple and the TiPb iPhone forums gi
[Twitter search, thanks angry complaining devs!]