The Minecraft server has been updated to version 1.3 and should be working correctly.
The information below has been retained in the event you're playing on an outdated 1.2.x server. Don't forget to download the new launcher. It's not necessary, but it is advisable.
Please read this before you run your client today (February 22nd).
If you haven't already done so, make certain to download the new launcher; this will give you the option to selectively apply updates.
If you have updated your client, you won't be able to connect to our server. The Minecraft forums are down as of this writing, and I haven't yet figured out how to update (converted) the server-side maps as updating it with the client seems to break things. For the time being, download the 1.2 client from our site and replace the contents of your %APPDATA%/Roaming/.minecraft with the contents of minecraft.zip. This should overwrite the %APPDATA%/Roaming/.minecraft/bin directory with the one under minecraft.zip/.minecraft/bin.
Be sure to make a backup of your existing .minecraft first and use the new downloader to launch your client.
Once the server is updated to version 1.3, you can either delete the %APPDATA%/Roaming/.minecraft/bin/version file to force an update or use the Options -> Force Update from the new launcher.
On *nix platforms, substitute %APPDATA%/.minecraft with ~/.minecraft. On Windows, this should typically be found under C:\Users\[username]\AppData\Roaming\.minecraft (Vista or later) or C:\Documents and Settings\[username]\Application Data\.minecraft (or C:\Documents and Settings\[username]\Local Settings\Application Data\.minecraft) for XP or earlier.