Is there a better way to travel internationally with an iPhone (3GS/4 specifically)? I've travelled to Europe and Asia quite a bit with iPhones with no problems (I live in the US), except for the fact that making calls anywhere outside the US is crazy expensive and of course I wouldn't dare turn on data roaming for fear of the thousand-dollar bill.

These days, though, is there not a better way? Can I not go into a shop in the UK and buy a pay-as-you-go micro-sim for 30GBP and call/data away? Is my iPhone (3GS and 4) permanently and forever locked to AT&amp;T unless I jailbreak it, even with a different sim card?

I'm willing to jailbreak my 3GS to accomplish this mission, but I'd like to hear from someone with real-world experience before doing so.

Thanks!
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.