Offline folders
 

I run Vista (RTM) and Office 2007 on my desktop; the computer is part of a
domain and the excel files are saved on the server (SBS 2003 R2).
I have several folders that are on the server marked as available offline;
my problem is that, when disconnected form the server, I can open every file
but the ones saved in the native Office 2007 format (xlsx). I tried to
create a test spreadsheet and save it in the same location and the same
name, one copy as test.xlsx and the other as test.xls; the the latter is
available when I am offline, the former not. Also note that both of them are
marked as available offline under "Properties"/ "Offline files" Tab.
Thanks in advance for your advice.
Stefano