I used two cisco 3750s, as the hosts. One with the IP address on the physical interface facing the access port on the MX80 and the second 3750 with the trunk interface, I created a SVI on the 3750 in the same vlan. I was able to ping from the SVI on one 3750 to the ip address on the 2nd 3750 via the MX80.
Hostname: MX-80
Model: mx80
JUNOS Base OS boot [10.3R1.9]
JUNOS Base OS Software Suite [10.3R1.9]
JUNOS Kernel Software Suite [10.3R1.9]
JUNOS Crypto Software Suite [10.3R1.9]
JUNOS Packet Forwarding Engine Support (MX80) [10.3R1.9]
JUNOS Online Documentation [10.3R1.9]
JUNOS Routing Software Suite [10.3R1.9]
Good luck