I'm having similar problems unfortunately. When I first bought my Macbook Pro, it worked fine on the Internet, but since then I have been compeltely unable to access any wireless networks. Admittedly, I haven't tried plugging it into the modem directly, but ideally I would like to get it running on the wireless. I am able to conntect to the network, but having attempted to access the Internet on several different wireless networks, i'm beginning to think that it is a problem with the Macbook Pro itself.
As far as I am aware, I haven't changed any of the Internet settings on the Macbook Pro. Other laptops are able to access the Internet through my current wireless connection, including my girlfriend who uses a regular Macbook. Is there anything I can try to resolve this?
By the way, I know that this isn't overly helpful, but I'm not sure which OS i'm using at the moment. I'm a recent PC convert, so I'm relatively new to all the world of Apple Macs!