Rails 3 and Haml breaks ERB

I installed rails 3.0 beta and haml 2.2.21 and created a new project,
and initalized haml for it. I then generate a standard ERb scaffold
(rails scaffold Person name:string) and add "gem 'haml'" to the
Gemfile.

When i run the server and head to People#new, I get the following:

<pre>undefined method `html_safe' for nil:NilClass

Extracted source (around line #1):

1: <% form_for(@person) do |f| %>
2: <%= f.error_messages %>
3:
4: <div class="field">
</pre>

I am really at a loss here. In a more complex project, where the
layout is rendered with haml and the form with ERb, the application
trace shows that form_for is dispatched to Haml:

<pre>
    /Library/Ruby/Gems/1.8/gems/activesupport-3.0.0.beta/lib/
active_support/whiny_nil.rb:49:in `method_missing'
    haml (2.2.21) lib/haml/util.rb:216:in `html_safe'
    haml (2.2.21) lib/haml/helpers/xss_mods.rb:118:in `form_for'
    app/views/schools/_form.html.erb:1:in
`_render_template_1729594521_2187273160_1077630'
    /Library/Ruby/Gems/1.8/gems/actionpack-3.0.0.beta/lib/action_view/
template.rb:40:in `send'
    /Library/Ruby/Gems/1.8/gems/actionpack-3.0.0.beta/lib/action_view/
template.rb:40:in `render'
</pre>

But I was not able to reproduce that trace with the clean project,
which only gives me a trace from within the app (for some reason). If
I remove haml from Gemfile and plugins, it renders without problems.

Here's the relevant parts of my gem list:
actionmailer (3.0.0.beta, 2.3.5, 2.3.4, 2.3.2, 2.3.1, 2.2.2, 1.3.6)
actionpack (3.0.0.beta, 2.3.5, 2.3.4, 2.3.2, 2.3.1, 2.2.2, 1.13.6)
actionwebservice (1.2.6)
activemodel (3.0.0.beta)
activerecord (3.0.0.beta, 2.3.5, 2.3.4, 2.3.2, 2.3.1, 2.2.2, 1.15.6)
activeresource (3.0.0.beta, 2.3.5, 2.3.4, 2.3.2, 2.3.1, 2.2.2)
activesupport (3.0.0.beta, 2.3.5, 2.3.4, 2.3.2, 2.3.1, 2.2.2, 1.4.4)
arel (0.3.1, 0.2.1)
builder (2.1.2)
bundler (0.9.11)
configuration (1.1.0)
crack (0.1.7, 0.1.6, 0.1.4)
daemons (1.0.10)
erubis (2.6.5)
gem_plugin (0.2.3)
gemcutter (0.5.0)
haml (2.2.21, 2.2.16, 2.2.14, 2.0.6, 2.0.5)
mail (2.1.3)
memcache-client (1.8.0, 1.7.8)
mongrel (1.1.5)
rack (1.1.0, 1.0.1, 1.0.0, 0.9.1, 0.4.0)
rack-mount (0.6.1, 0.4.7, 0.4.0)
rack-test (0.5.3)
rails (3.0.0.beta, 2.3.5, 2.3.4, 2.3.2, 2.3.1, 2.2.2, 1.2.6)
railties (3.0.0.beta)
rake (0.8.7, 0.8.3)
RedCloth (4.2.3, 4.2.2, 4.1.9, 4.1.1)
rubygems-update (1.3.6, 1.3.5, 1.3.1)
sqlite3-ruby (1.2.5, 1.2.4)
thor (0.13.4)
tzinfo (0.3.17)
xml-simple (1.0.12)

For the record, I have now also tried with rack-mount 0.4.0 (ie.
removing rack-mount 0.4.7), and it made no difference.