IOS 5.01 as you might have seen is an extensive operating system with varied functions and as apple claims it has more than 200 new functions.The excessive data usage in the Iphone 4s was reportedly due to the default "switched on" status of the iCloud service and it shipped with the IOS 5. Now even if you don't use this service there might be applications(such as games) which might be loading advertisements which usually end up eating a whole lot of data than you know.You are provided with free games and applications only because you are a source of revenue which they get through you by showing you their advertisements

From what I see right now is that you need a good network data usage software for your Iphone 3GS which sorts out application-wise data usage and you can find the culprit(application ofcourse)and kill it so that you can use your data for much vital purposes.