~ $ cd /tmp
 ~ $ ln -s /usr/local/bin/python .
 ~ $ ./python -c "import sys; print sys.executable"
'import site' failed; use -v for traceback
/tmp/python

Here's the traceback, I'll look at the problem asap

'import site' failed; traceback:
Traceback (most recent call last):
  File "/MacDev/svn.python.org/python-trunk/Lib/site.py", line 530, in <module>
    main()
  File "/MacDev/svn.python.org/python-trunk/Lib/site.py", line 509, in main
    addbuilddir()
  File "/MacDev/svn.python.org/python-trunk/Lib/site.py", line 118, in addbuilddir
    s = "build/lib.%s-%.3s" % (get_platform(), sys.version)
  File "/MacDev/svn.python.org/python-trunk/Lib/sysconfig.py", line 577, in get_platform
    cfgvars = get_config_vars()
  File "/MacDev/svn.python.org/python-trunk/Lib/sysconfig.py", line 402, in get_config_vars
    _init_posix(_CONFIG_VARS)
  File "/MacDev/svn.python.org/python-trunk/Lib/sysconfig.py", line 255, in _init_posix
    raise IOError(msg)
IOError: invalid Python installation: unable to open /usr/local/lib/python2.7/config/Makefile (No such file or directory)

proposed fix.

actually realpath == abspath on most platforms where symlink is not implemented.

Changed.

Why would sys.executable be a directory?

Antoine, see http://bugs.python.org/issue7774

Yes, it is the issue #7774.

The patch proposed on msg98284 should fix sys.executable being a directory.

Tarek, if you implement the patch for sysconfig, add a note regarding the dir hack:
  # XXX : see issue 7774

Fixed in r78828 and r78829 (py3k), except for the part related to #7774.

(It was turning buildbots red, after Victor fixed #3137)