Consider the following project structure:
# spec/my_lib_spec.rb
require "rspec"
describe "should work" do
it "with rcov also" do
MyLib.should be
my_method.should == "works"
end
end

# lib/my_lib.rb
module MyLib
  def my_method
    "works"
  end
end

require "rspec"
RSpec.configure do |config|
  config.include(MyLib)
end

# .rspec
-r
lib/my_lib 

# Rakefile 
require 'rspec/core/rake_task' 
RSpec::Core::RakeTask.new(:spec)
RSpec::Core::RakeTask.new(:rcov) do |spec| 
  spec.rcov = true 
end 

When using rake spec then example passes, but with rake rcov i see
this:
C:\Users\jarmo\Desktop\minu\projects\Ruby\blah>rake rcov
(in C:/Users/jarmo/Desktop/minu/projects/Ruby/blah)
c:/ruby/bin/ruby.exe -S rcov -Ispec "./spec/my_lib_spec.rb"
F
Failures:
1) should work with rcov also
Failure/Error: my_method.should == "works"
undefined local variable or method `my_method' for
#<RSpec::Core::ExampleGroup::Nested_1:0x59cb488 @__memoized={}>
# ./spec/my_lib_spec.rb:6
Finished in 0.014 seconds
1 example, 1 failure
rake aborted!
ruby -S rcov -Ispec "./spec/my_lib_spec.rb" failed
(See full trace by running task with --trace)

When i set a breakpoint into the example block then with rake spec i can
see that module is included, but not with rake rcov although
my_lib.rb is loaded because the first expectation succeeds as seen
above.

With rake spec:
(rdb:1) l
[3, 12] in C:/Users/jarmo/Desktop/minu/projects/Ruby/blah/spec/
my_lib_spec.rb
3 describe "should work" do
4 it "with rcov also" do
5 MyLib.should be
6 require "ruby-debug"
7 debugger
=> 8 my_method.should == "works"
9 end
10 end
(rdb:1) self.class.included_modules
[MyLib, RSpec::Matchers, RSpec::Core::MockFrameworkAdapter,
RSpec::Core::Pending, RSpec::Core::Let::InstanceMethods,
RSpec::Core::Let, RSpec::Core::Subject,
RSpec::Core::Extensions::InstanceEvalWithArgs, RSpec::Mocks::Methods,
PP::ObjectMixin, RSpec::Core::SharedExampleGroup,
RSpec::Core::ObjectExtensions, Kernel]

With rake rcov:
(rdb:1) self.class.included_modules
[RSpec::Matchers, RSpec::Core::MockFrameworkAdapter,
RSpec::Core::Pending, RSpec::Core::Let::InstanceMethods,
RSpec::Core::Let, RSpec::Core::Subject,
RSpec::Core::Extensions::InstanceEvalWithArgs, RSpec::Mocks::Methods,
PP::ObjectMixin, RSpec::Core::SharedExampleGroup,
RSpec::Core::ObjectExtensions, Kernel]

For some reason the module is not included when running with RCov.