MacRumors reports that many AT&T customers in the US have been finding that they are getting much improved speeds on AT&Tís 3G network on their iPhone 4Ss. This is because the new iPhone is able to take advantage of the much faster HSDPA support for GSM networks where offered, and at present, MacRumors reports, only AT&T is able to take advantage of this, so AT&T customers are definitely onto a winner with their new iPhone. According to MacRumors, AT&T is usually intentionally vague about what sort of speeds its network is capable of, but MacRumors says that two of its readers in New Jersey and Iowa reported download speeds of 7.71 Mbps and 5.45 Mbps respectively, much higher than on the iPhone 4. Not everyone has experienced the new speed boost however, so it probably depends on what part of the country you live in as to whether or not youíll benefit. Either way, as MacRumors notes, it proves that AT&Tís network is currently capable of much faster speeds than either Sprint or Verizon.
Source: Some AT&T Users Seeing Much Improved 3G Speeds with iPhone 4S - Mac Rumors