I am a developer and had these issues also, there are no notes stating that you have to install xcode beta first. After following some of the suggestions on developer forum and using tiny umbrella, I was able to install 4.2b. After that I was able to restore several times just fine using xcode beta. By the way the response time to a bug report is usually 48hrs and I don't want to wait that long.
Procedure I followed:
1. completely uninstall all previous versions of xcode.
2. restore iphone4 to 4.1 in itunes(fresh download not GM) (got error 10)
3. use tiny umbrella to kick out of recovery.
4. install xcode beta
5. use itunes to update to 4.2b (got error 10)
6. use tiny umbrella to kick out of recovery (iphone 4 is now at 4.2)
7. I was then able to test a few applications and restore several times using xcode beta without any issues
Not sure what the issues are, but I have had problems just restoring some of my test phones(various IOS versions), have at times had to restore on my pc, maybe it is time for a fresh install of OSX.