I'll be traveling to England, Prague, Vienna, and Budapest. (Thinly veiled brag) What's the best way to go about getting service, including data, on my HTC Sensation? Will I need a new SIM card in each country?
If you have a phone that is unlocked and can take any GSM, then you can always buy prepaid sims to save the hassle of worrying about costs for international calling. The question is if you are going to use your phone for stuff in Europe or if you want to call back home.
It needs data. I would recommend that you get a prepaid card in every country, this will be the cheapest option for you. Can be done without too much hassle, usually right at the airport when you arrive.