You may have noticed that Minecraft 1.3 was released by Mojang and the server is still 1.2.5. The bad news is that we are stuck on 1.2.5 until bukkit updates to 1.3 and it may take even longer if it takes more time to update the plugins. So if you want to log in you’ll have to get the 1.2.5 client and install it.
First go here and get the minecraft 1.2.5 .jar file: http://www.avrgaming.com/downloads/minecraft.jar
Then goto start and in the search box type: “%appdata%” (if you are on windows XP, click on “run” and type it in there)
Browse to the “.minecraft/bin” folder and replace the minecraft.jar file with the one you just downloaded.
You should be downgraded to 1.2.5 and able to login.
Thanks for your patience guys