tcpdump is required for netdude operation, so shouldn't it depend on it in the ebuild? I had to go do a seperate "emerge tcpdump", which isn't that big of a deal, but portage should do it for me. I was building netdude-0.4.6, which consequently is the only one available (and it is marked with the ~x86 keyword).
Fixed in CVS thanks for pointing it out :)