Since apple has not released any specs and have not divulged any specific details, there cannot be any definitive answer to your question. My advice is to wait until June 7th and then see if apple addressed your question
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Chances of AT&T actually having a reasonable LTE network next year are very low anyways. You might be lucky to get spotty coverage at best if you're in one of the primary cities.
the iPhone 4 will probably just support HSPA+. The 3GS supports HSPA. ATT won't rollout full HSPA to the entire country for atleast 1-2 years. HSPA+ should be rolling out to 250 million Americans later this year.....