Specifically, from Comcast:
http://www.comcast.com/Corporate/Customers/Policies/HighSpeedInternetAUP.html
Technical restrictions
"use or run dedicated, stand-alone equipment or servers from the Premises that provide network
content or any other services to anyone outside of your Premises local area network (“Premises
LAN”), also commonly referred to as public services or servers. Examples of prohibited
equipment and servers include, but are not limited to, email, web hosting, file sharing, and proxy
services and servers; "
Will they complain and notify you?
Probably only if you start to generate a significant amount of traffic.