Yes. If any of your users are using Word 2000, the mail merge main document
will probably fail to open (and may "hang" Word). if the data source is
already in use. With Word 2002 and later, they will probably see at least
one dialog box saying that the file is already in use and offering to open a
read-only copy. If they choose that option, they may be OK, but there may be
additional problems (and for example they wouldn't be able to make any
updates to the data source).

IMO if you are going to have a data source shared between multipe users, you
should really use a data source that is designed for multiple simultaneous
users - an Access .mdb is OK for a small number of simultaneous users,
particularly if the data is intended to be read-only, but a proper
client-server DBMS (such as SQL server, or even its cut-down version) is
preferable as the user count increases.

--
Peter Jamieson
MS Word MVP