There is no way Apple is going to ask 3gs owners to pay for iOS 6. The reason iPod users were charged was due to some accounting differences in the way devices were classified.
That said, I don't think 3gs will get iOS6. iOS5 already chugs and the 3gs will be replaced by the iPhone 4 as the entry level iPhone.
That said, I don't think 3gs will get iOS6. iOS5 already chugs and the 3gs will be replaced by the iPhone 4 as the entry level iPhone.