I created a separate no site-packages virtualenv directory, and use easy_install to install Reviewboard.
However, easy_install installs PIL version 1.1.7 which does not work with the latest stable Reviewboard version 18.104.22.168.
Using easy_install PIL==1.1.6 doesn’t work because of build error. The workaround is to manually download PIL and install it.
To make reviewboard work under virtualenv, additional steps are needed, which in fact are steps for Django application to work under virtualenv.