Normally we don’t give much credence to a single tweet pertaining to rumors about the next iPhone, but this one comes from Eldar Murtazin, editor-in-chief of mobile phone blog Mobile-Review. Murtazin has a history of accurately predicting mobile phone rumors and in this tweet says the iPhone 3GS will not be upgradable to iOS 5 when it debuts this fall: “Just one comment. Apple iPhone 3Gs wont be upgradable to iOS 5.x. iPhone 4 will.” [Note that it is possible that Murtazin was referring to the plural of iPhone 3G — 3Gs — and not the iPhone 3GS model, but it’s not clear. –Ed.]

Apple dropped support for the original iPhone in iOS 4 and limited support to certain features of iOS 4 on the iPhone 3G, so some might think it’s obvious that iOS 5 will not support the iPhone 3GS — a two year old phone by the time the new operating system ships. However, as MacRumorspoints out, Apple still currently sells the iPhone 3GS and not supporting the newest OS on something they currently sell (at least until the day they announce the new iPhone) would be out of character for the company. But with all the rumors that iOS 5 will sport the biggest changes iOS has seen since its debut, it’s conceivable that iOS 5 might have some pretty big hardware requirements and could require at least an A4 chip and possibly even a Retina Display to run.

Source: http://www.tuaw.com/2011/05/24/ios-5-rumor-iphone-3gs-to-drop-off-supported-list/