a much simpler method for non-jailbroken iphones:

just log on to this website from your iphone browser: http://unlockit.co.nz/

follow the instructions and it's done.

to reactivate, go to Settings on your phone -> General -> scroll to the bottom and delete the Fake APN profile.

Yup I can concur with tinkerbel. I'm on the 500mb package too and haven't gone overboard for the last 2 months. Oh but actually for the last month, I went a little too happy on the internet tethering (accessing net from client's office for work), and by the 15th of the month my counter showed me a total of 504 mb download and upload together. I immediately applied the fake apn profile like I mentioned above for the rest of the month. Anyways, just got my bill, and luckily I wasn't charged for the 4mb extra.

Anyways moral of the story, u should be alright keeping below the limit, just watch yourself if you're tethering too much And the data counter on the iPhone may not be the most accurate, but it's good enough. Just give it a rough error margin of 5 to 10 mb and it'll be fine.