If you logged into Minecraft today, then you either took the chance of updating to 1.5.2, or remained on the previous revision. For those who didn’t update, congrats. For those who did update, don’t fear.
The server will be updated as soon as Bukkit and other major plugins get imported over to the new version. While we do this, the server will remain on version 1.5.1.
To download the previous version so you can still play, download the minecraft 1.5.1 jar from here.