Minecraft has been updated to 1.5.2! This is a big change, as you cannot connect to a 1.5.1 server with a 1.5.2 client. Unfortunately, Bukkit has not been updated to a 1.5.2 beta or recommended build yet. When this happens, we will let you know, and there will be an option to update your server from the settings area of the user panel! Thank you for your patience!