I was on a paintball server when I first noticed this, trying to join a team (quite crippling). No commands work, no messages work, nothing. Whenever I type anything I am greeted by "Cannot send chat message".
This even applies to "singleplayer" (on a world where cheats are allowed).
Couldn't exactly ask others for help, on any server...
The only mod I have installed is Optifine.
Edit: Deleting everything in /bin and redownloading didn't fix it.
Makes me think it's some authentication issue.
Edit2: Deleting /.minecraft did in fact fix this. But that doesn't make a whole lot of sense for the bug to be outside of /bin. Curious to know if others have gotten this or what the source is.