_my_partial.html.erb

  <% if my_block_content.present? %>
      <%= my_block_content%>
  <% else %>
    <%# default without block %>
  <% end %>

my_helper.rb

module MyHelper
  def special_thing(locals, &block)
    content_for_partial = capture(&block) if block_given?

    render :partial => 'shared/my_partial',
           :locals => locals.merge(:my_block_content => content_for_partial)
  end
end

template_using_helper.html.erb

<%= special_thing({:a_local_variable_for_partial => some_object.value}) %>

<%# OR with a block %>

<%= special_thing({:a_local_variable_for_partial => some_object.value}) do %>
  <p>Some <em>special</em> markup</p>
<% end %>