Cha-cha-cha

There have been reports in FT and elsewhere of people inadvertently running up cell phone charges in the hundreds or even thousands of dollars, especially when traveling internationally. I don't understand how this is possible, probably because though I have an international cell phone, I never use it for much except an occasional voice call or text message -- it's almost always turned off.

Can someone explain to us novices how it's possible to rack up such huge charges without realizing it?

Is there any way to protect yourself from this?

If you have a netbook or laptop connected to the internet via the cell phone network, does the same risk exist? Any protections?

Thanks for any enlightenment.