*UPDATE*
After working on this all day long, it appears that the problem is OS X is not sending the proper AD Credentials to the scripts on login. This is causing the SMB authentication error. Is there a work around or a fix for this? Here is the part of the script we are using:
/usr/bin/su -l "$3" -c "/sbin/mount_smbfs "//servername/location/toshare/${3// /%20}/desktop" '/Users/$3/Desktop'"
As you can see we are running the command as the currently logged in user and we are getting an authentication error...this worked perfectly with 10.7.4...
I'm also having this issue. Though I think it is a Kerberos change in 10.8. It looks like even with a valid ticket, it still prompts me for a password in terminal. Kerberos should allow this to mount without authentication.
Running it as a login policy is your problem. Because you're realistically running as a different user (root or the casper admin account) OS X prevents access to the user's issued Kerberos tickets. This is a security feature, not a bug.
Run your script as a launchd item instead and it will work fine. (This is what I'm doing.)