Legolas visited the Bellevue Square Lego store and came back with this…

When working with Sharepoint and attributes that you want to make available, keep in mind that Sharepoint uses a Global Catalog search on port 3268/3269 rather than a standard LDAP query on port 389/636.  However, not all attributes in the AD schema are a part of the Global Catalog attribute set.

This means that if the attribute you’re trying to search for isn’t in the GC then Sharepoint won’t see it until you add it to the list of attributes in the AD schema that should be a part of the GC set.

Example: the serialNumber attribute in AD

Add an attribute to the global catalog
http://technet.microsoft.com/en-us/library/cc737521(v=WS.10).aspx

All Attributes [in Active Directory]
http://msdn.microsoft.com/en-us/library/ms675090(v=vs.85).aspx