That could be, as Win7 really should be run on Server 08, Kernal's are different than 2003.
But have you checked the local systems to make sure they are setup to run the login script?
You have to set the GP on the local machine to look for and run the logon script I think, You can't just set it on the server alone.
My GP skills need improving really. But if the local GP is not set to run a logon script, then that could be the problem.
I guess the real question is, do you have other things in the logon script that are running fine, but not mapping drives?
goto local pc and run gpupdate /force and let it reboot, maybe it's just not pulling it and your systems have not had a full GP update to the local pc.
Can you post your logon script and omit any private info with XXXXXXX in it's place?
Now that I think about it a bit, you say you map them as persistant but when you login, they are gone. Something is running and removing the mappings.
Maybe you have a GP conflict, are you doing any inheriting? blocking? These things can turn into a nightmare.