Hi, all. First bug report here, so please let me know if I can be more
helpful / less verose. I didn't see this previously reported on the mailing
list and I'm not yet familiar enough to patch it myself.

After updating, I'm receiving the error in the subject line. For reference,
I'm running Arch and am using the latest org from elpa. So:

Emacs  : GNU Emacs 24.3.1 (i686-pc-linux-gnu, GTK+ Version 3.6.4)
 of 2013-03-18 on eric
Package: Org-mode version 7.9.4 (7.9.4-elpa @
/home/trevor/.emacs.d/elpa/org-20130318/)

To produce the error, I start a fresh instance with emacs -Q, then run the
following:

(add-to-list 'load-path "/home/trevor/.emacs.d/elpa/org-20130318/")
(setq org-babel-load-languages '((python . t))) ; also works with latex, C,
ruby, at which point I got bored
(require 'org)

I expect this to load just fine, but instead I receive a backtrace that
freaks out at line #39 in ob-python.el (similarly for other language
examples):

38     (defvar org-babel-tangle-lang-exts)
39     (add-to-list 'org-babel-tangle-lang-exts '("python" . "py"))
40

Seems that the defvar counts for nothing? Perhaps because it's already a
defcustom in ob-tangle.el? I don't understand emacs well enough to know,
but I will say that inserting a "(require 'ob-tangle)" anywhere before the
offending line "solves" the problem.

Trev