I have just received a message from o2. I didn't actually think I had unlimited data? I signed up for a new contract on the day the iPhone 4 came out.

O2: Your unlimited data promotion ends on the date of your next bill. Which is when your new 750MB a month data allowance starts. Last month you used more than that. For more info or to find out how to buy more data visit http://go.o2url.com/data

It's just as the text message explains.

All customers had unlimited data until their first bill after the 1st October.

At that point, any customer that changed tariff after June 24th (The Day the iPhone 4 was released), would have a limit applicable to their tariff applied.

In your case, that's 750MB.