yes, you can get in trouble.. by trouble, I mean you can get charged for useage and or possibly have your contract cancelled. I've heard some cases of this.

I have SprintPCS Vision service, and within my 'Unlimited Vision' (browsing the net from the phone) plan, I get (duh) unlimited internet through my phone without a change in my monthly bill. I bought a USB cable and connect quite easily.. and browse quite heavily. I've even downloaded a song or two in the past few weeks.. been using the phone as a modem for about 4 months now and no charges have been on my bill. I'd imagine it's different with non-unlimited plans, but perhaps in my case, I'm being overlooked as an 'unlimited' user.

I'm certainly not going to contact them to tell them my situation

good luck!