When I get to Chapter 6 and start working in sessions, the
registration form page breaks. I get this error message:

ActionController::InvalidAuthenticityToken in User#register

 Showing user/register.html.erb where line #2 raised:

No :secret given to the #protect_from_forgery call.  Set that or use a
session store capable of generating its own keys (Cookie Session
Store).

Extracted source (around line #2):

1: <h2>Register</h2>
2: <% form_for :user do |form| %>
3:      <fieldset>
4:              <legend>Enter your details</legend>
5:

RAILS_ROOT: /home/www/ruby/rails_space
Application Trace | Framework Trace | Full Trace

/usr/lib/ruby/gems/1.8/gems/actionpack-2.1.0/lib/action_controller/
request_forgery_protection.rb:115:in `form_authenticity_token'
(eval):2:in `send'
(eval):2:in `form_authenticity_token'
app/views/user/register.html.erb:2:in
`_run_erb_47app47views47user47register46html46erb'

[snip]

I have tried the errata suggestion: comment out the line with
the :secret key in app/controllers/application.rb.

That change has no effect.