I have this issue as well. The original post doesn't seem to make clear if just having mumble open prior to launching TF2 causes the crash, and for me it doesn't. TF2 will only crash if I have the overlay enabled. I can provide the error thing that OSx provides of crash if that's at all useful to you.
edit: I tried the overlay with Minecraft and it works. The overlay works in Half Life 2, another source engine game like TF2. From my understanding it's an older version of the source engine, so I tried with Portal 2 (which is a more recent game). The overlay function works fine there.
Perhaps the October 26th patch has something to do with the crash?http://wiki.teamfortress.com/wiki/Octob ... 2012_Patch
That update added positional audio support for Mumble clients. This was also the exact day I started using mumble and attempted to use the overlay. Maybe the implementation of positional audio in TF2 is related to this? I don't understand why it wouldn't affect other source engine games though. Enabling or disabling positional audio has no effect on the crashing of TF2.
I don't really know what I'm talking about though, I doubt any of this has anything to do with the real cause.