If I use a prepaid SIM card from AT&T retail store in iPhone, do I get 3G or GSM service? I have an officially unlocked iPhone 4 from a non-US carrier. Now that I'm in US, if buy a prepaid SIM card from AT&amp;T and use it in iPhone, do I get 3G(WCDMA) or GSM network? To put it in the context, do I see "3G" or "E" in the iPhone status bar?

Could anyone clarify if this chart is correct?

Enable 3G, Enable Cellular Data, Result
Yes      , No                  , Make 3G call;  use no data
Yes      , Yes                 , Make 3G call;  use 3G data
No       , Yes                 , Make GSM call; use EDGE/GPRS data; 
No       , No                  , Make GSM call; use no data;
Presuming that it's not a CMDA iPhone (i.e. not from Verizon/Sprint), you will see '3G' if you're in a 3G coverage area. The Enable 3G and Cellular Data options must be turned on in the Settings.

Sidenote: AT&amp;T requires purchasing a data package for their smartphones; no Pay per use.

As AT&amp;T also had a GSM network, you could also access that (and GPRS/EDGE as well).

Usual Terms and Conditions will probably apply.