I am using WebDAV queries against Exchange Public queries. Because MAPI folders do not support deep traverse, I've used Multiple Folders as it is shown in MSDN :
FROM SCOPE('shallow traversal of "http://myserver/public"',
'shallow traversal of "http://myserver/public/userinfo"',
'shallow traversal of "http://myserver/public/userinfo/phonenumber"')
And I beleive that it worked against Exchange 2000 (I couldn't confirm it, because I have now only Excahge 2003 available).
But against Excahge 2003 default(AKA MAPI) Public Folders the query returns not user-friendly
System.Net.WebException: The remote server returned an error: (501) Not Implemented.
And after second reading I noticed that it is documented:
Note Searching multiple folders by specifying multiple scopes in the query is not supported on a MAPI enabled public store.