According to Brian Komar, CDP and AIA extensions won’t work any more with file://\\server\share URLs as of Windows Vista SP1 / Windows Server 2008.
Note: With the release of Windows Vista Service Pack 1, support for Common Internet File System (CIFS) or Server Message Blocks (SMBs) through a File URL was dropped for AIA and CDP retrieval.
From Windows Server 2008 PKI and Certificate Security, page 245.
Why post it here? Because I couldn’t find this information on the interwebs, only in a book. I spent 20+ minutes looking!
A book. In this day and age!?
Aha! Here's the KB article describing the change! http://support.microsoft.com/kb/946401
(if like me you got hung up on SMB or CIFS being a keyword for the change, well, there ya go. I guess that file:// just implies whatever you've got going on in your redirector, so while SMB/CIFS might be most common in Windows networks, it could have been NCP/NFS/Whatever here.)
Hope that saves you some searching, future-Tristan.