File locking while using offline folders

I have a folder on our server (Windows server 2003) setup to work offline on
my Vista notebook. I was working in this folder, with status of 'online'. I
had a Word file open all morning. Someone in the office then opened the file.
They were _not_ informed that the file was already open, instead, they were
permitted to save changes. When I later went to save changes I was told that
the file was in use.

Does anyone know what is going on here? I thought that working online meant
that the system would have the file locked for me. Is there a way of ensuring
that this happens?

(Using Vista business sp1)