arpspoof

I appologize if this is the wrong place to be posting this. I am
running mac os x 10.3.9. I installed dsniff 2.4 through darwin ports.
When I try to run arpspoof (e.g. sudo arpspoof -i en1 -t 192.168.1.101
192.168.1.1), it acts as though it is working, but doesn't actually do
anything. When I try sniffing with ethereal, I can see that there is
no arp traffic that is being generated. My friend does the same
command on his os x 10.4 ibook and it works fine. Any ideas?

-John