Well, it all looks right to me. I can't see anything out of the ordinary. The last thing I would try is changing to this:
If DCount("*", "tblCustomerComments", "[CommentAlarm]= #" & Format(Me.txtCommentAlarm, "mm\/dd\/yyyy) & "#") > 0 Then
and if that doesn't work then perhaps if you post a copy of your database (with bogus data of course) maybe David, since he has the same regional settings, could try it out and see if he can spot what is wrong.