Update: Valve have chimed in:
A fix for this has just been released. Your clients should automatically download it. If not, as an alternate workaround, try unplugging all unnecessary USB devices – this crash bug is primarily triggered by alternate input devices, like tablets.
It’s been… six months since the last time this happened. Steam is crashing for many Mac users on launch, as part of an “update” process. It seems to be affecting users who are enlisted in the
Trading Cards beta programme, but information is a bit sketchy at this point. Downloading a new version from the website doesn’t work, and no workarounds have been found.
There’s a (very small) thread on the official forums about this, and as usual, not a peep from Valve by way of acknowledging the problem. We’ve contacted the digital distributor for an update, and in the event we get a reply, (1) we’ll update this post, (2) hell will have frozen over.