To clarify: these are messages (text or symbols) that appear right at the point of interaction, and then float away and fade out.
You see this kind of animation when adding people to your Circle in Google+. A green +1 symbol floats up and disappears. You can see this for yourself using the Google Plus Demo without having to have a Google+ account.
Here on Stack Exchange, when you visit a chat room, your avatar floats gently down from the top of the screen and disappears, presumably as a notifcation that 'someone has just dropped in'.
It's common practice in platform games and immersive environments where points scored, or 'new life' symbols appear and then float away.
And we're starting to see this more on television where occasional animated fading text floats away from the relevant person. The BBCs Human planet and Sherlock are some examples.
My feeling is they are: more engaging compared to common status bars and message areas; contextual because of their localized positioning; flexible because of their free movement and independence from page structure and layout; not too invasive due to their transient nature.
But perhaps not so good in situations where you may not easily have the opportunity to repeat or re-live the prior sequence in order to see what happened after the notification has faded away, since once it's gone, it's gone.
So when is it appropriate to use this mechanism?