Is there a better way to travel internationally with an iPhone (3GS/4 specifically)?
As you've pointed out, US iPhones are locked to AT&amp;T. Fundamentally, that's your only real issue here. If you can unlock your iPhone, then your suggesting of just picking up a micro-sim and using it for data and calls works perfectly. In the UK, we can buy iPhones unlocked, and get our network operators to unlock them.

What you described is exactly what I've done when travelling abroad, but it's entirely reliant on having an unlocked device to start with. From there, you should be good to go. In the UK, there's a massive range of low-cost, pay as you SIM only options to choose from. O2 do a big range of SIM only deals, including micro-sim plans (see here).

The other option to consider is, if you're looking to upgrade to an iPhone 4 anytime soon, buy it abroad, from the UK, unlocked, and then you won't have to jailbreak it in the future. UK and European iPhone models will work perfectly well in the US, and they come with a global warranty as well.