Minecraft 1.6.4 has been released today (yes, .4, not .3), and it's not compatible with the current server version of 1.6.2. As such, you need to stay at the previous version in order to continue playing with us!

In the launcher, click the "Edit profile" buttton and select "release 1.6.2" as the "Use version" setting. You will then be able to join as usual. A further news post will be made once we update.

I recommend selecting the current version number from the launcher even if we are running the latest one available. That way, whenever the client gets an automatic update, you won't have to go back and change your settings!