In regards to the Outlook 2003 and Shared calendars/accounts. Recently ran into this problem and discovered that setting the 2003 Accounts to "Cached" mode on the client, allowing it to build the OST and then restarting the client (thats 2 restarts after setting the check box), I was able to access the folder.
My testing included setting up a MAPI profile to access the shared account directly to verify permissions.
Once that was done, it was just trial and error till I found the right combo.
This is only a stop gap for our environment, as Cached mode is not something we want to do on our desktop systems so would appreciate a resolution. However I dont think the problem is going to be on the exchange side, but more on the office client side.
Our upgrade to Office 2010 is coming next fiscal year anyway, so it may be a moot point.