Hi Luc

I'm not sure if I understand the problem correctly, but maybe this
information will help:

In an Exchange server environment, when you look at a shared calendar,
you're looking directly at the information in that mailbox.

When you look at free/busy information (on the Scheduling tab when you
create a meeting) you're looking at a subset of calendar information
published to a hidden public folder.

In an environment with many Exchange servers, different users may be set up
to access different public folders.  In this case, Public Folder Replication
synchronises the information between public folders on different servers so
everyone *eventually* sees the same information.  But it isn't instantaneous,
and if the replication breaks then different people will see different
information.

http://www.msexchange.org/tutorials/FreeBusy-Folders-Exchange-Server-2003-Depth.html
has a much better explanation of how this works, so if the problem is related
to the free/busy public folder then hopefully this will point you in the
right direction.