Suggestion #1.rb

it 'should do something' do
  on_failure warn_that "@foo shouldn't be nil" do
    @foo.should_not be_nil
  end
end

Suggestion #2.rb

it 'should do something' do
  explain_that "@foo shouldn't be nil" unless
    @foo.should_not be_nil
  end
end

Suggestion #3.rb

it 'should do something' do
  on_failure do
    @foo.should_not be_nil
  end.report "this message"
end

Suggestion #4.rb (Aslak)

it 'should do something' do
  lambda do
    @foo.should_not be_nil
  end.on_error "this message"
end

Suggestion #5.rb (rjharmon)

describe "needs external software", :detail => 2 do
  describe "needs imagemagic", :detail => 3 do
    describe "convert binary" do
      it "should be found" do
         ...
      end
      it "should have png support" do
         ...
      end
    end
  end
end