Master Page behaviour in MOSS and WSS

Having single master page for all the sites?

If using MOSS, you can inherit from the top level master page. If you are using WSS, the out of the box functionality is that all sites utilize their own master page. You can, however, create a site template that uses the master page you wish and create all subsequent WSS sites from your template(s).

Master Page Inheritance
Master page files can be applied on a site level, but not on a page level. So a sub site can have a different master page specified than the parent, or it can inherit the master page file settings from the parent.