Problem
When trying to use facebooker according to the Developing facebook platform applications with rails book, in the network_test step you are getting the following error:

FBML Error (line 5): illegal tag "body" under "fb:canvas"

Solution
It should be caused because you are using the facebooker gem instead of the plugin.
Install the plugin:

ruby script/plugin install git://github.com/mmangino/facebooker.git