When I signed up to the Partner Program, Philip, as a 'non-US person', there was a load of tax stuff to complete and sign in blood, but since then it has been all straightforward, as they simply hand me the cash and the responsibility for declaring it appropriately here in Spain (which I'm still not sure how to do, but need to find out by June 30...).

I guess if you are a 'US person', i.e. citizen of or resident in the USA, the paperwork burden comes at a different point in proceedings.