Well, if the walls aren't all the way up yet, I'd just pick a closet where you could throw up a 19 inch telecom rack and stuff a patch panel in with enough ports to run one or two to every room (it's usually pretty easy to find RJ45 sockets that are integrated in to electrical face plates or similiar). Then simply wire the house with a cable or three to each room (numbered and labelled of course) and stuff a twenty-four port hub or something in next to the patch panel.

If you plan it right, you can even get a small rack-mountable PC in the closet and a DSL modem or something... install your firewall on the PC and hook it up to the DSL line and away you go. APC makes some pretty nice rack mountable UPS' as well.

You can even grab another machine or three (rack mountable, of course) and run a webserver or three... *shrug* DON'T forget the ventilation fans, though... the more machines you get in to that cramped space, the more you'll surely need it (you might even need to look in to AC for the closet, depending).