Possible Work Around
Not saying this is a 'FIX' but it works better than having to edit the DNS everytime you switch networks.
First, I just purchased a new iPad (iPad 3) and I have an iPhone 4 and MBP 2011 model. I was testin Facetime between all three, both inbound and outbound from each. Couldn't get the iPad to receive any incoming. It would ring once then notify me on the other device they are unavailable.
Out of curiosity, I decided to create a separate Facetime account for my iPad using another email address I have on hand. Now this isn't changing anything regarding my Apple account, just adding another Apple id to the primary account.
To Fix:
Go into Settings -> Facetime -> Create new apple id
So my iPhone uses the phone number to receive inbound Facetime, my MBP uses my primary email for inbound, and my iPad uses a separate email.
All works flawlessly now. A little complicated, but works better than having to edit DNS every time you connect to a new network (Home, Work, School, Coffee shop, Hotel, etc)