I've got a Nokia N73 and somebody told me that I can use it as a modem for my laptop, but I'm not sure how to set this up. It would be really useful because I'm already on a flat-rate data plan and I ...

You can use the cell phone as a "regular" modem, but you'll have to check your service plan, etc. I have an older Motorola StarTAC and a CF card that allowed me to do that with my Pocket PCs. Still ...