That's a shame - a lot of companies still use public folders extensively.
Especially disappointing since the old IMAP / OWA based Exchange support in Mail DID support public folders in Exchange 2003.
It looks as though Exchange Web Services does provide the ability to access public folders, so this is purely down to Apple not implementing it:
"Public Folder Access
In Exchange 2007 SP1, you can use the Exchange Web Services operations to access public folders. This includes the following functionality:
Access to the public folder hierarchy
Access to detailed information about public folders
Access to detailed information about public folder items
The ability to post items
The ability to synchronize public folder items
Subscription notifications for public folders"