Anyone facing any problem updating the iOS 4.2.1?
Many have complaint about it updating halfway and stopped.
Then your iPhone kept booting over and over again saying that the updating failed.
The phone appeared to be in Restore mode showing the iTunes logo with the USB cable.

Here's the solution for all:

On Mac:

* Open Finder
* Hold down COMMAND + SHIFT keys and press ‘G’
* Enter “/private/etc/” in the field and press “Go”
* Find “hosts” file in the directory
* Drag the file to your desktop
* Open it in text editor
* Remove the line that has gs.apple.com entirely or put a # at the beginning of it to comment it out
* Save the file
* Drag it back to the /private/etc/ folder.
* You’d need to enter your username and password to authenticate the move

On Windows:

Locate the hosts file in one of the following directories:
Windows 95/98/Me c:windowshosts
Windows NT/2000/XP Pro c:winntsystem32driversetchosts
Windows XP, Vista, 7 c:windowssystem32driversetchosts
Right go and go on “Open With…” and next visit on “Notepad” on the list.
* Remove the line that has gs.apple.com completely and save the file