Minecraft 1.6.5 has just been released!
As you may be aware there were a number if issues with the 1.6 release. Versions 1.6 to 1.6.4 all had various issues.
Notch has just released version 1.6.5 which hopefully will solve these issues.
Please make sure to upgrade to 1.6.5 if you haven't yet!
If you need help upgrading, please open a support ticket.