I just tried to install ROS (Electric) on OSX10.7 on my machine. I ran into two hurdles. Here are the fixes: 

1: rosinstall fails, says no yaml python module.
Cause: rosinstall is using the wrong python (/usr/bin/python), pyyaml was installed with macports.
Solution: edit /usr/local/bin/rosinstall, replace "#!/usr/bin/python" with "#!/usr/bin/env python". 
Ticket: https://code.ros.org/trac/ros/ticket/3660

2: rosmake fails, with a compiler segfault in rosconsle
I was disappointed to find that this is a known bug, with a known fix, and has been known for weeks prior to release. I'm sorry to see that a version of ros shipped with what is essentially a fatal issue in its core library. It seems this is a bug in Apple's llvm-gcc, and I know that OSX is not officially supported in ROS, and that there are a lot of hard issues that WG doesn't have time to support. This one takes a few minutes, and the workaround while not ideal seems harmless. A simple #ifdef __APPLE__ could have even made the workaround only for OSX platforms, leaving Ubuntu unaffected. 
Here is the bug/fix: https://code.ros.org/trac/ros/ticket/3626
It works. Thanks William for the workaround.