Looks nice, but my first thought when I looked at the code example:
1 >>> l = ['spam', 'ham', 314, 23]
was "wait, how are they assigning a list to the number 1? How is that possible? Oh... that's an l". (Not a fan of that. I'm half blind (figuratively) so these things confuse me.)
+1 to this. I was drawn to Code Academy because I could immediately start playing around with code on the home page. (At least it used to be like that. Not sure anymore.)
I'd suggest improving the colour scheme of the code window, dark green on dark grey background is not very legible. Maybe steal some colors from the Sphinx doc colour scheme?
Yep, that was my first reaction as well. However, the idea of preview can be extended, codeacademy style. Make a small python interpreter in js, that people can try in, that gives them an interactive tutorial for python.