Have a look at your Broadband contract, but on the whole if your router is doing
NATing, they will normally not notice anyway.
Personally I would say network away, after all with the rise of multi computer households and games consoles like the XBox 360 and PS3 that connect to the internet I don't think the ISPs can really complain. After all you cant use any more bandwidth than your pipe allows can you?
Jen