I was curious about this since I started tethering my Curve yesterday and have already had it connected for about 10 hours.
I checked my account online at AT&T and saw that my data plan is called Internet/MEdia Net and it said I have Unlimited access and already used 78.8 mb of the unlimited. I still wanted to double check and called AT&T and they confirmed I have unlimited modem access.
So I am happy about this.
I suggest if you have a question to check online as well as calling AT&T.