The simple fact that all of today's info is post on the At&t site on the IPHONE INFORMATION PAGE should spell it out.
The prices shown on that page are for TWO iPhone 3G lines.
If you have 1 iPhone 3G and 1 regular phone, it will be exactly the same as if you had 1 WinMo phone and 1 regular phone, or 1 BlackBerry and 1 regular phone...
I will use my plan (1 normal phone and 1 BlackBerry) as the example.
2100 shared minutes- $100
2nd line- $9.99
Unlimited family text- $30
BlackBerry Personal Plan- $30
Total = $169.99
If I swap the BlackBerry out for the iPhone 3G, NOTHING CHANGES. It will be the exact same cost.
Again, the Family Plan info on the "iPhone information" page is for TWO iPhone 3G lines.
Same 2100 minutes on 2 iPhone 3Gs...
2100 shared minutes- $100
2nd line- $9.99
Two iPhone 3G data plans- $60
Total = $169.99
Get it?
If you want to get an iPhone 3G and a normal phone on a two line Family Plan, all you have to do is add $30 to the cost.
What he ^ said.