Have you lost your personal hotspot settings on your iPhone after either updating to iOS 5 or getting an iPhone 4s? Well I got it back by resetting the network settings. This worked for me but you do this at your own risk.
What I did was tap on settings then tap on General, scroll down to the bottom and tap on Reset, tap on Reset Network Settings, you will then be prompted for a passcode if you have one (a good idea to have one), enter passcode, tap on Reset Network Settings. Your iPhone should now restart. Be patient as it will restart and could take a couple of minutes. I also had to wait for a moment before it found the mobile phone network and I had to reenter the password for my wifi, so make sure you have that.
As I said this worked for me and I know it has worked for others but you do this at your own risk.
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