Disclaimer: This post does not imply that the Internet Sharing feature will only work if you have the Spanish keyboard installed. It only implies that the build on the Windows Phone 8 devices handed out at //Build/ had an issue that prevented users on the ATT network to use Internet Sharing. My guess is that by adding a keyboard (any keyboard probably) to the device we are also triggering an update that solves the Internet Sharing issue.
Update (11/26/2012): If the issue happens again installing another keyboard does the trick (I used Vietnamese).
I was very excited to receive a Nokia Lumia 920 while attending the //Build conference last week, but was extremely disappointed that the Internet Sharing feature was not working even though my ATT plan supported it. Obviously, that was one of the first things I wanted to do since I wanted to share the Data Plan on the device with my brand new Microsoft Surface. Well, by sheer luck, I was able to get it to work just follow the instructions bellow to enable it on your //Build Nokia Lumia 920 device:
1) Go to Settings
2) Go to Keyboard
3) + Add Keyboards
4) Select Spanish (Spain)
5) Click on the checkmark icon to install.
The only reason why I found this workaround is because I write a lot of messages to my family in Spanish and I hate the fact that the English keyboard tries to autocorrect Spanish using an English dictionary as I type. But, little did I know that it was going to have this wonderful and unexpected side effect.