ATT has not specifically said (as they did when the iPhone came out
last year).
If you're a current AT&T customer and upograding from a non-iPhone to
an iPhone 3G. ATT vaguely states you need a qualified Voice Plan.
I was told my $69.98 2 person family plan 850 minutes,
9 PM to 7 AM Nights is such a Qualified plan. Just $30 more per month
for data for each iPhone 3G on the plan, and extra is I want txt
messaging. No need at all for messaging with such wonderful email
capability, not to mention Chat clients.
Current voice plans for new customers only provide 700 minutes
at my price points, and Nights end at 6 AM.
Keep current ATT voice plan for iPhone 3g 
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