commands are as follows:
[no] ip inspect name L2-transparent dhcp-passthrough
p26-24.12(config)#ip inspect ?
L2-transparent Transparent Mode commands
...(existing inspect commands)
This command enables the DHCP passthrough behaviour in the feature. New features like ARP inspection will follow the L2-transparent keyword along with dhcp-passthrough.
http://www.cisco.com/en/US/tech/tk331/tk660/technologies_tech_note09186a0080094471.shtml