use 3GS as modem tethering without giving up unlimited plan iPhone

No.
How long have you had the unlimited data plan and what is the largest amount of data you have used in a month?
Have you ever used more 1GB of data in a month or anywhere close to it?
 MacBook Pro 15" Intel Core 2 Duo/2.53GHz/4GB RAM/500GB HD Mac OS X (10.6.4)  iPod w/Video 30GB,  iPhone 4 32GB

I think that my current phone usage is irrelevant considering that I do most of my surfing on my home network right now. My usage will surely show significant increase if and when I start using my phone as a modem. Somebody told me (a customer service rep from att) that there is a way, but he could not give me specifics.
Message was edited by: tater_jake
imac 20"/powerbook g4 Mac OS X (10.4.10)

The tethering plan is completely separate from the iPhones data plan so it is not irrelevant, but I can answer it for you - you havent used over 1GB of data in a month or anywhere close to it which is why you dont want to answer the question.
The iPhones data plan required with the separate tethering plan includes 2GB of data for the iPhone, which you would be hard pressed to exceed or get anywhere close to. The tethering plan and limit is completely separate which applies when tethering only.
Somebody told me (a customer service rep from att) that there is a way, but he could not give me specifics.
Too funny. Since the only way this would be possible is with and by AT&T, I wonder why this AT&T customer service rep could not give you specifics?
 MacBook Pro 15" Intel Core 2 Duo/2.53GHz/4GB RAM/500GB HD Mac OS X (10.6.4)  iPod w/Video 30GB,  iPhone 4 32GB