Ahem. There have been multiple exploits for heroku, some of which enabled access to code and data of other heroku customers (google for "heroku vulnerability").
From what I read about their virtualization (which may not be up-to-date) they seem to rely on the security of chroot(). If that is still the case then there is a big problem in their future.
chroot has not been designed as a security feature but as a system testing tool. you only need a local root exploit to get out of chroot. you need additional protection to have a proper jail; freebsd does this, openbsd used to, not sure how it is now.
That is definitely interesting, although for Ruby that is slightly simpler because there is no real way like PHP to have a single instance of Ruby deal with all of the requests (talking about mod_php and PHP-fastcgi, not php-cgi which spawns a process per request).
