example.rb

class Example
  class << self
    def attribute
      @attribute ||= nil
    end
  end
end

spec1_spec.rb

require File.dirname(__FILE__) + '/example'

describe Example do
  context "Running spec1" do
    it "attribute should be nil" do
      Example.instance_variable_get(:@attribute).should be_nil
    end

   it "attribute can be set in this spec" do
      Example.instance_variable_set(:@attribute, "an attribute").should == "an attribute"
   end 

    it "attributes can be gotten in this spec" do
      Example.instance_variable_get(:@attribute).should == "an attribute"
    end
  end
end

spec2_spec.rb

require File.dirname(__FILE__) + '/example'

describe Example do
  context "Running spec2" do
    it "attribute should be nil" do
      Example.instance_variable_get(:@attribute).should be_nil
    end
  end
end