I was pounding my head against the keyboard. Rspec with rcov was crashing whenever it felt like it. I was getting all kinds of errors like:

/opt/local/lib/ruby/gems/1.8/gems/rspec-1.1.11/lib/spec/matchers/be.rb:92: [BUG] rb_gc_mark(): unknown data type 0×0(0×489ffc8) non object
ruby 1.8.6 (2007-09-24) [i686-darwin9.3.0]

opt/local/lib/ruby/gems/1.8/gems/rspec-1.1.11/lib/spec/example/example_group_methods.rb:344: [BUG] Segmentation fault
ruby 1.8.6 (2007-09-24) [i686-darwin9.3.0]

not to mention a bus error or two as well.

Then I found this article with this solution:

$ gem sources -a http://gems.github.com (you only have to do this once)
$ sudo gem install mergulhao-rcov

from this guy.

Then the world was right again.