Tom,
Most rooms, such as kids rooms usually get one. Master bedrooms get one or two, plus a couple CAT5 next to the bed. Typically I will put at least 2 hybrid cables in the home office and one or two in the kitchen. I dont work for production builders anymore because they would limit the amount of runs, ie: Four multi-ports in a 3500 sq ft home and advertise "wired for the future" Custom builders are more open to covering all the bases and doing it right. In new constuction I also run at least one 2" conduit from the basement to the attic for future pulls. I see that alot of BIG companies ( like SEARS ) are getting into the game which will put pricing pressure on guys like us, we will have to build value with knowledge, expertese, and quality of the install ( like everything else we do ) Structured wiring has become a huge part of my biz and got me alot of A/V work too. Do your homework before you jump in and you will do fine. Good Luck!!
Deco