Shortly after we moved into our current house we finished part of the basement which soon became my office. With my wife's computer upstairs and mine downstairs I could either go wireless on one or more of the computers or run cables. I decided to run cables since I had access to large amounts of Cat5e cables and running the cables wouldn't be particularly hard by using the cold air return as a wire chase.
I don't know of any problems with running network cabling this way. It seemingly should be pretty safe. Since it's cold air returning to the furnace, there won't be any temperature issues and it should be isolated from any other electrical interference. The only difficulty may be getting into and out of the return. I'm not sure if it's really allowed by building codes, but the cable I am using does happen to be plenum rated.
If you are not aware, plenum rated cable is designed to run in the ceilings (plenums) of offices. Normally used to circulate air in a building, plenums should contain cables that are designed not to give off toxic fumes if it were to burn. Of course the wire cost more and personally I don't think it really would make a difference as there are plenty of other materials in the typical building to give off toxic gases other than network cables.
We are looking at buying or building another house hopefully in another 5 years or so and I think I would also go with a wired home. I would go with Cat5E or Cat6 and if needed throw a wireless router onto the network for any laptops.
