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