First thing to try - Does your unix box have a default gateway of the pix?
Second thing -
I wasn't clear - when you VPN in you get an IP that is not local to the network. The ASA/PIX houses that internally and routes accordingly. You did this with the IP pool command. Now the unix box may not respond to things not on the local network (ie your new VPN ip pool) due to a local firewall, ipchains, whatever on that box. This is the only thing I can think of.
IP is IP and shouldn't react differently due to OS.
Brent
Systems Engineer / Consultant
CCNP, CCSP