[Samba] exec scripts question

[ipguy]

i'm having problems with preexec scripts... see example below

[homes]
preexec = mkdir /samba/users/%U

do exec scripts only work in a Samba PDC setup or are we able to use exec
scripts in any setup ?
if you look at the preexec script above, all i'm trying to do is make a dir
for the connecting user (i don't need to make an account as the samba server
is seup as a domain member server to a NT4 PDC, i'm using winbind for
accounts)
why wouldn't the script work ?
it's to simple for it not to....