Well besides Siri, the other big reason to upgrade from an Iphone 4 to a 4S is international travel...it is a dual mode CDMA/GSM phone, so potentially you can use the very good Verizon network here (or Sprint if the coverage is good for you and their prices are much, much better). Here are the wrinkles according to Maclife and Pocketnow

• You can jailbreak and unlock the phone and play cat and mouse 🙂
• One user on Macworld says, "The I-phone has realistically always been a world phone. I travel for a living and have been tricking my phone to work with local sim cards for years. Having done so with my I-Phone orignal and 3Gs. The process is as simple as buying a pay as you go SIM card from any Offical Apple Wireless Carrier... think O2 in the UK, Orange in France, etc... Then simply activate this SIM in the corresponding countries I-Tunes store. when I-phone first came out this only worked in 5 countries but every year Apple adds new markets and it has been working in all those places for me. Very little known fact but works everytime. And yes, sometimes navigating in a different language is tricky and makes the process less than ideal, but you save alot of money." Personally, I haven't tried this but it would be terrific if true. Next time I'm in Australia I will have to try it. You are right, Apple is in many countries.
• You can buy a Verizon plan and they will internationally unlock the phone for any foreign SIM after 60 days. Sprint GSM unlocked phones right now, but will later lock it and then you will have to request an unlock (strange, strange).
• You can spend $649 and buy an Apple unlocked phone but this only works for GSM (!!?) in November and never have to worry about this or having a contract every again 🙂 Since most plans are two years ($600!), this actually might make sense if you have say good coverage with T-Mobile and can use their lower cost (although slower speed) plans.

And here is a price comparison (of course for those of us on family plans, migrating requires flipping all the phones over!), but it shows Verizon and AT&T are basically the same price, so it just comes down to network while Sprint is much cheaper for SMS and data but more expensive for voice but coverage is the issue: