Back when Notch was doing the coding he acknowledged the bug that lets you turn redstone into obsidian but said he "might fix it if competitive play takes off" or some such, but it seems to be forgotten and very unlikely to change now. It seems like a plugin could fix it, though. I know you could still generate obsidian from nether portals but it takes a lot more effort so I'm not really concerned about it.
I do, but are you sure it has that feature? It doesn't seem to be mentioned in the documentation: http://wiki.sk89q.com/wiki/WorldGuard/Configuration/Bukkit
