Mojang released Minecraft 1.6.4 today. This version is kind of a weird one. According to the release notes, its only purpose is to prepare for the upcoming 1.7 release. See the JDRGaming Minecraft page for details.

The JDRGaming Minecraft server has been updated from 1.6.2 to 1.6.4.

Note: My initial attempts to join the server failed with a mysterious error. I started a single player game using an existing world without any difficulty, so I again tried to join the JDRGaming server and this time it worked. Unfortunately, the error was not logged anywhere on the client or server end, so I don't know exactly what it was. However, if you have trouble connecting to the JDRGaming MC server, it might help to try what worked for me.