the retail license applies only to a single machine - but is transferable, meaning you can install it on a new pc as long as you wipe the disk of the old one
i think microsoft are planning a family pack license for multiple pcs, if you order from their recently-announced online ordering things (apple have done this for mac os x for a while)
edit: aha! http://windowsvistablog.com/blogs/wi...nts-today.aspx
online ordering, and $49.99 per home premium license if you buy ultimate retail (upgrade or full retail but not oem)