What could cause a Minecraft portal to not work?

Are you in single player mode? Portals don't yet work on multiplayer servers.

Specifically, there is a multiplayer server command, "hellworld=true" that is applicable for testing, but is buggy and not yet fully implemented.

As of 1.6, Nether Portals do work in SMP. Make sure you're updated to 1.6 or later if you're trying to access the nether in SMP.

Portals don't currently work in multiplayer because of the way the code was written. In the single-player game, when you enter a portal, the normal world gets unloaded and the Nether world is loaded instead. The same process happens in reverse when you return from the Nether.

On a multiplayer server, the normal world can't just be unloaded willy-nilly, since other players may still be playing on it! This means that for portals to work correctly the server would need to have the normal world AND the Nether loaded at all times and that's currently not possible.

Presumably this is because the server code was originally written with the assumption that there is only one world loaded (or possibly there are just memory issues with having both loaded at once), so until Notch refactors the server code to allow for both to be loaded at once, we're stuck with no portals in SMP.

In 1.7, check your server.properties file and make sure the nether option is set to "true".