The PIX doesn't understand trunks. It examines the VLAN tags of any traffic on an interface and discards any traffic tagged for VLANs it has not been configured to understand.
The lines below are from a config I have used for over 1 year.
PIX Version 6.3(4)
interface ethernet1 100full
interface ethernet1 native_vlan_id physical
interface ethernet1 vlan2 logical
interface ethernet1 vlan3 logical
nameif ethernet1 NAME1 security_xx
nameif vlan2 NAME2 security_yy
nameif vlan3 NAME3 security_zz
after this is done you can add the IP addresses etc