Copying this 2.1.1 bugfix to the trunk:

Fix showstopper SF bug #442983: use of site.addsitedir() was broken
because it references the global dirs_in_sys_path which is deleted.
The fix avoids deleting that global.

(My email through python.org or digicool.com is non-functional at the
moment; use gvanrossum@home.com to reach me.)

Lib/site.py

@@ -151,7 +151,6 @@ for prefix in prefixes:
             if os.path.isdir(sitedir):
                 addsitedir(sitedir)
 
-del dirs_in_sys_path
 
 # Define new built-ins 'quit' and 'exit'.
 # These are simply strings that display a hint on how to exit.