Not sure why people are bothering to complain about the supposed small print. Go to the O2 website and choose Tariffs. All the iPhone Tariffs are down as being unlimited but with an asterisk after them which says "fair usage policy applies".
I'm complaining because 200MB is by no means "fair usage". It's barely "normal". Usually, fair usage has a limitation which is above and beyond "normal" use, and hints more at "abuse" of the service.
To say a service is unlimited, when normal usage from the majority will likely go over the 200MB fair usage is misleading, irresponsible, and down right tardy!