For what it's worth here's what the AT&T CSR replied to my email enquiry.
Mr. John Doe, at initial launch, you will only be able to purchase the 3G
iPhone at company owned retail stores (and select kiosks) and Apple
stores. At that time, your phone will be activated. You will not go
through the iTunes account to activate the new 3G iPhone like you did
with your current iPhone. While your phone is being activated, the
representative will apply the new contract and the new data plan at that
time. The new iPhone data plans do not include messaging. It only
includes unlimited internet/email. At this time there is no information
regarding the text messaging feature.
You did understand correctly regarding your rollover minutes. If you
keep the same rate plan, you will retain all of your rollover minutes.
Note: My emphasis added.
Mr. John Doe, at initial launch, you will only be able to purchase the 3G
iPhone at company owned retail stores (and select kiosks) and Apple
stores. At that time, your phone will be activated. You will not go
through the iTunes account to activate the new 3G iPhone like you did
with your current iPhone. While your phone is being activated, the
representative will apply the new contract and the new data plan at that
time. The new iPhone data plans do not include messaging. It only
includes unlimited internet/email. At this time there is no information
regarding the text messaging feature.
You did understand correctly regarding your rollover minutes. If you
keep the same rate plan, you will retain all of your rollover minutes.
Note: My emphasis added.