This is new, though I have heard reports of it before - I hadn't experienced it until this week. Occasionally, you try and send a tell to someone and World tells you it cannot find the client. Tells have always worked for me, but Scatman reported once a few weeks ago he suddenly couldn't send tells anymore - after we had been using tells all afternoon.
Now that it's happening to me it's an issue.

haha well also, Nalandial reported it yesterday as well, so I knew it wasn't just poor Scat.
This is using the latest dev SVN (as of this post). I am not sure if this is post-TSO or before. Maybe Scat can remember the first time he started seeing the problem.