Is there a way to make portals for teleportation in SMP?

There are a few plugins available if you run the Bukkit mod.

Here's an example, though there are still bugs associated with the Nether in general.

There is a bit of a read here, but it is all pretty relevant.

Since the above is all pretty Nether-related, and you said you want portals in the same world, check out this as well.

Portal functionality is not yet implemented in SMP. You can make the portal and light it, but stepping inside will do nothing.

It is unsure exactly how SMP portals will be implemented, but Notch has been quoted saying that they will link different servers together, and he confirms that they will be usable for fast-travel within a server. Common rumor also states that they will allow transportation between SMP servers and single-player worlds.

When you create a world in SMP, you can either create it as a normal world or a hell world. Once it has been created, it is not recommended that you switch between types, as problems can occur. Be sure to set hellworld=true or hellworld=false inside server.properties before allowing the server to run.