Posts controller...
...
#I want to allow only the post owner to be able to view "show" their own post(s)

...

# this will work
def show
    @post = current_user.posts.find(params[:id])
    if @post.nil?
      flash[:notice] = 'Access Denied'
      redirect_to :action => 'index'
      return
    end
    respond_to do |format|
      format.html # show.html.erb
      format.xml  { render :xml => @post }
    end
end

# I would do this though
  def show
    @post = current_user.posts.find(params[:id])
    (render(:nothing => true, :status => 403) && return) if @post.nil?
    respond_to do |format|
      format.html # show.html.erb
      format.xml  { render :xml => @post }
    end
  end

... other basic crud methods

...