Hi David;
I have used my Active Directory web service for this. However, while this works great for Windows and Web that gets the AD Signature from the ADSI API - of course in the mobile world, you cannot do that as the AD signature API is not available.
My workaround was to capture the AD Signatures in a DB table. This table was basically a list of authorized users on the mobile system along with their AD signatures (encrypted like passwords for security). My Mobile Apps would take the User ID at login, lookup their AD Signature in the DB table, Decrypt it, then pass that along to the Web Service for login and App privileges based on AD Group rights.
Food for thought!
Regards ... Chris