What happens if I go over my T-Mobile data limit?

I have an iPhone 4 on an internet data plan of 750mb per month. I was wondering If I go over my limits what are the charges?
Thanks.

Added (1). So there's NO CHARGES? You just get limited to browsing only, I definitely don't have to worry about being charged?

Look what it is http://m.guardian.co.uk/technology/blog/2011/jan/14/t-mobile-data-explanation