_post_form.haml

%dl
  %dt
    =f.label :title, :caption => "Post Title"
  %dd
    =f.text_field :title, :value => @post.title, :size => 50
  %dt
    =f.label :body, :caption => "Post Body"
  %dd
    =find_and_preserve do
      =f.text_area :body
  %dt
    =f.submit "Submit"

app.rb

# the Post.new here, doesn't give the "body" property any content, so, nil.
  get '/new_post' do
    authenticate!
    @page_title = "Add Post"
    @post = Post.new
    haml :new_post
  end
  post '/new_post' do
    authenticate!
    @post = Post.create(params[:post])
    redirect '/'
  end

# but this will create an empty, and correct, tag:

  get '/new_post' do
    authenticate!
    @page_title = "Add Post"
    @post = Post.new(:body => '')
    haml :new_post
  end

edit_post.haml

-form_for @post, "/edit_post/#{@post.id}", :method => 'POST', :class => "admin-form" do |f|
  =partial 'post_form', :object => @post, :locals => { :f => f }

model.rb

class Post
  include DataMapper::Resource
  property :id,           Serial
  property :title,        String,   :length => 255
  property :slug,         String,   :length => 255
  property :body,         Text
  property :created_at,   DateTime
  property :updated_at,   DateTime
end

output.html

<!-- 
   here the textarea tag is prematurely closed, which 
   Firefox doesn't seem to like. You can see the result in 
   this screenshot:
   http://farm5.static.flickr.com/4029/4207451802_188bc71a76_o.png
-->
<textarea name="post[body]" id="post_body" />

THanks for the excellent code, HAML, and http://farm5.static.flickr.com/4029/4207451802_188bc71a76_o.png shot!