Quite often (but not always) when I send a message from Playbook I'll end up with two copies in the message list, one 'checked' sent-mail copy and one green unfinished copy. When I select the green copy it says 'this message has not been sent' even though it has.
I have had the same experience as well. At first I thought it was simply a "Twitter issue" because I noticed it when replying to an inbox message but then it happened again when attempting to send a Gmail message.
I think this is a pretty common problem, as I'm actually having problems with the feeds as well being read in the contact book. I think that their is a delay with Facebook of some sort right now, hopefully RIM recognizes the problem.