When requiring simplecov in a Rails 2 project, I get an uninitialized constant error due to ::Rails::Railtie. If I comment out the part that loads the railtie.rb file, code coverage generation works fine. Maybe you could define the Rake task in a different way if using Rails 2 or simply refrain from defining it if RailsTie is uninitialized?
Damn, that's a big regression. Will fix it today and release a new gem.
Only load Railtie if Rails 3 actually available in form of Rails::Rai…
Just pushed 0.6.1 to Rubygems.org, should resolve this.