I've downloaded drupal 7.24 and petitions 7.x-2.0-beta19. Everything seemed to worked smoothly during installation and a clone of the Whitehouse Petitions website is available.
However, petitions cannot be created since a 404 error is produced when trying to access "/petitions/create" within the iframe.
I've seen that this very same issue has been reported before (#48). However, the issue appears as closed and this pull request is provided as a solution (#49).
Unfortunately, changing the file wh_petitions.form.inc with the version provided in pull request 49 does not solve the problem but, instead, produces a new one. The whole page is loaded into the iframe and the petition is still not created.