I've been thinking about this. Since you really don't want to run wires through walls (understandable), I think the choices are wireless or Power over Ethernet (PoE). For PoE, you would need PoE adapters and I.P. video cams. You would have to setup port forwarding at your DSL modem/router (not hard). With wireless, you would need a base that could handle, 4, 8, 16.... cams... Doable. There is a Motorola device that can be used..
Motorola Homesight Gateway - No PC Required Version - XG1000 - X1000 - SmartHomeUSA.com
There is also a system that allows you record the video feeds and also watch live via the Internet.
Xanboo DVR for HomeSight Gateway - XD925 - SmartHomeUSA.com
(back ordered)
BTW, these homesite systems allow you to hook up other things, such as light controllers, thermostat.... It's more of a home automation system, but would work for your needs too.
If you have a dynamic external I.P., then you would need to sign up for a free service like
DynDNS.com: Free DNS Hosting, E-mail Delivery, and VPS Hosting