We now have a temporary 1.5 server running. This server will be online until the Bukkit team has an updated server software, and everyone has creative on this server. On the live server, only the nether world will get reset, because of the new nether quartz in the world.
This means, the server on which you have played already, with all the worlds etc. will be back online after the guys from Bukkit have an updated server software.