Can I use iPhone 4 3G services outside of my native country? I have an iPhone 4, bought at my home country and I have a data plan with a local telco carrier. My phone has never being unlocked. See, I am such a nice citizen in Apple's walled, seclusive and closed Kingdom!

Now, I want to travel overseas and I am prepared to get a local sim card with 3G data plan wherever I go. My question is, can I use iPhone 4 with 3G services? What should I do to do so? Do I have to unlock my phone first? Will that render my warranty invalid?
The UK uses the GSM 900 / GSM 1800 MHz bands. These are, among others, supported by the GSM iPhone 4.

From the iPhone 4 - Technical Specifications page:


  GSM model: UMTS/HSDPA/HSUPA (850, 900, 1900, 2100 MHz); GSM/EDGE (850, 900, 1800, 1900 MHz)


These bands should give you coverage in most of the world, providing that you get a simcard from a GSM provider:

GSM World Coverage