I have Bukkit on my SMP server, and world portal so I can have multiple worlds. The problem is, when I build a nether portal, pig zombies spawn next to it, but when I go in it, it just plays the spiral animation and doesn't teleport. I think it has to do with my server having World Portal installed. (And yes, allow-nether=true in server.properties)
It sounds like you have no Nether world for your new world. World Portals takes over all portal and other-world generation, so you need to tell the server to make a Nether so that Nether portals will go to it. Basically, you need to follow this sequence of actions:
It should now create a new Nether world, and your Nether portals should work!