@blstz,
If you're not doing heavy download u shouldn't have to worry.  I'm on the 500mb p/month plan and thus far haven't had any issues with my data usage + bill.  Am waiting for this months' bill to double confirm but should be fine.

My last bill apparently states 23MB of usage! *gulps* But that's not accurate since I swapped plans then so this months' statement should be more accurate, even then I don't expect it to hit 100MB since my i3GS Usage only shows:

Sent: 30.8MB
Received: 105MB

And that's since I started using the i3GS.

@lotus_man.
SMS does not constitute as data service under "Usage"; receiving MMS on the other hand will form part of it.

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.