iOS 4.1 fixed proximity, Apple offered a free case to anyone who had an iphone 4, i hae one and the signal issue wasnt even an issue. It didnt affect me whatsoever.
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imo even for technology 3 months isn't a long time to wait, especially if you already have something that works fine for the moment. This culture of having to update your phone every year is just encouraged by the manufacturers when a new phone now should easily be able to last 4 years if not more. Consider that you already have an iphone which is not different enough to the iphone 4 to be worth an upgrade.
A phone is a phone. Your 3GS probably works fine for everything it's made for. iPhone 4 does the same apart from that tiny bit higher spec and facetime which is just glorified video calling.
Another reason to wait and get the iPhone 5 is doing one of your unboxing vids and making money from it on your youtube channel.