I would like to know if it is possible to access 3G on my UK HTC Sensation when I go to New York next month on a U.S. SIM card? My limited understanding is that the frequencies used for U.S. phones are different to those in the UK but am unclear if the Sensation will still be able to access. If it is not possible could I just use a US SIM for voice calls and rely on WiFi access?
Great question, with several good answers. Basically, yes, you can use your UK model HTC Sensation for 3G data in New York. The European Sensation uses a quad-band GSM radio that supports the frequencies used by AT&T here in the states. You'll need to make sure your Sensation is fully SIM unlocked (talk to your current carrier if you're unsure), and then you'll be ready to do a little research and make a decision.
In the U.S., there are only two GSM operators -- T-Mobile and AT&T. But there are many MVNO networks (Mobile Virtual Network Operators) who rent and resell network space from either one, or even both. You'll not be interested in T-Mobile or any MVNOs using the T-Mobile frequencies, as your Sensation doesn't support them. But that's OK, as your choices are still pretty broad.
Here's a small list of a few different operators that will sell you a no-contract SIM card to use while you're visiting:
Don't be fooled by the words unlimited, as this refers to voice calls. Data rates will cost anywhere between $5 USD for 5MB to $20 USD for 2GB. Of course, this is just a few of the many out there, and you'll have to scour the web to see all the options. In the end, they all use the same network, so the deciding factor is price and how easy it is to get your SIM card and activation.