I can't say for sure when it started, but it seems to be since the last update. I'm finding the Steam client will regularly just sieze up. The store browser continues to work, but things like notifications of new forum posts don't appear. It'll often (but not always) have 'no connection' at the bottom of the client window, even though the Internet connection is funtioning normally. Restarting Steam fixes it all, but only if I force quit - trying to quit Steam normally causes it to go into perminant beach ball and just lock up.
Edit: Forgot to say, my best guess on any regular causal event for this is MacBook going to sleep. It seems to work fine up until the point I close the lid, or the computer goes to sleep on it's own. Once I come back to the machine, it seems not to work any more, and freezes when I try to quit.