Index: pkg_resources.py
===================================================================
--- pkg_resources.py	(revision 53773)
+++ pkg_resources.py	(working copy)
@@ -125,14 +125,10 @@
 
 def _macosx_vers(_cache=[]):
     if not _cache:
-        info = os.popen('/usr/bin/sw_vers').read().splitlines()
-        for line in info:
-            key, value = line.split(None, 1)
-            if key == 'ProductVersion:':
-                _cache.append(value.strip().split("."))
-                break
-        else:
-            raise ValueError, "What?!"
+        import plistlib
+        pl = plistlib.readPlist('/System/Library/CoreServices/SystemVersion.plist')
+        value = pl['ProductVersion']
+        _cache.append(value.strip().split('.'))
     return _cache[0]
 
 def _macosx_arch(machine):