You see, when you "broadcast" the cable connection, you are opening it up for anyone to potentially use it. So other people can potentially get Internet access from Comcast without paying for it. In Maryland, for example, it is illegal to use an "unlawful telecommunication device" which is a "device, technology, [or] product . . used to provide the unauthorized . . . transmission of . . access to, or acquisition of a telecommunication service provided by a telecommunication service provider." Delaware, Florida, Illinois, Michigan, Virginia and Wyoming all have laws on the books that may do the same thing.
These laws generally treat "sharing" of Internet connections the same way it would treat "sharing" of Cable TV or Satellite TV services. Thus, while you could invite your neighbors in to watch the latest episode of The Sopranos, you probably couldn't hook a coax into apartment 3B so they could watch from home -- at least without getting the permission of the cable TV company.
You can see this in, for example, Verizon's personal DSL agreement, which states that "[y]ou may not resell the DSL Service, use it for high-volume purposes, or engage in similar activities that constitute resale (commercial or non-commercial), as determined solely by Verizon." So, if Verizon determines that your 802.11 connection constitutes a non-commercial resale (and is unauthorized) not only can it cut you off, but it can make you a felon.
All of this means that the simple act of driving around and getting WiFi connections as needed, something we hope to be able to do (isn't that why we bought the Centrino in the first place?), is fraught with legal risk.