Forgot all about this thread. Hope you got it going. If not try this
check postfix main.cf and make sure your subdomain is listed. I think it needs to be defined in virtal_alias_domains. never tried it with a subdomain so not too sure on this
/etc/postfix/virtual
[email protected] babaganoosh
then edit your /etc/aliases
babaganoosh: "|/usr/bin/php -q /path/to/your/script"
Then run the following commands
postmap /etc/postfix/virtual
/etc/init.d/postfix restart
newaliases
This works on Ubuntu 10.04
Edit: You do not need to set up a system user for your alias.