Going abroad with your smart-phone? Understand roaming! Nowadays there's no excuse to be 'ignorant' of the colossal charges you could incur if you access the internet from your phone whilst in another country; there are countless reports of people being hit with thousands of pounds of charges during a week or fortnight holiday.
Your smart-phone will have a 'Data Roaming' on-off switch in its settings. On the iPhone, go to 'Settings: General: Network'. If you don't know how much you will get charged for using data services abroad, just swipe this option to 'Off'.
Even if you disable data roaming, you can still access the internet with wi-fi services in places such as hotels and restaurants, and that's almost always free of charge.
If you decide that you need data roaming (eg, accessing Google Maps whilst in the middle of nowhere), you should know the charges. These charges will vary (sometimes considerably) from one country to another. Your mobile operator may have a package deal that could make it a lot cheaper for you (in the order of £0.10 per MB). An important point is that from July 2012, within the EU anyhow, charges will be limited by law to £0.68 per MB - so you shouldn't get hit too hard anyway.
Your mobile operator has these responsibilities too (EU customers only, travelling within the EU):
You could also consider getting a special SIM that has billing customised for international use - but that's probably only worthwhile if you travel a lot.