Nether Portals seems broke at least on SMP. I own a server and travelled through a nether portal from the overworld to the nether. It shall be said (though I don't know if it matters) this portal were not in the spawn chunks.That worked fine.
When I went through another portal back to the overworld th game crashed. This portal was in the spawn chunks. When I try to log into the server I can briefly see that I am on the overworld side of the portal, but only for about a second and then it crashes again.