Some American projects declare new work to be in the public domain, before its copyright would normally expire. Does this mean the work can be treated as public domain here?
Yes usually. If the person making the declaration is in a position to declare a work public domain in the US (i.e. they are the author and control the copyright) then they are in a position to do this everywhere else. However if they have assigned their copyright outside the US to someone else then, obviously, this is not the case.
