Let me know if I can provide any more info.

C:\Users\wsargent\work\trunk\manager>rcov --version
rcov 0.8.1.2 2007-11-22

C:\Users\wsargent\work\trunk\manager>ruby --version
ruby 1.8.6 (2008-08-11 patchlevel 287) [universal-darwin9.0]

:\Users\wsargent\work\trunk\manager>rake spec:rcov:unit

** Invoke spec:rcov:clobber_unit (first_time)
** Execute spec:rcov:clobber_unit
rm -r doc/rcov/unit
** Execute spec:rcov:unit
............................................................/spec/unit/controllers/application_spec.rb:80: [BUG]
rb_gc_mark(): unknown data type 0x2c(0x1c8e8c4) non object
ruby 1.8.6 (2008-08-11) [universal-darwin9.0]

rake aborted!
Command /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby
-I"/Users/wsargent/.gem/ruby/1.8/gems/rspec-1.2.8/lib" -S rcov --exclude "spec/*,gems/*" --rails
-o "doc/rcov/unit" "/Users/wsargent/.gem/ruby/1.8/gems/rspec-1.2.8/bin/spec"
-- "spec/unit/controllers/application_spec.rb" "plus many more files" failed
/Users/wsargent/.gem/ruby/1.8/gems/rspec-1.2.8/lib/spec/rake/spectask.rb:177:in `define'
/Library/Ruby/Gems/1.8/gems/rake-0.8.7/lib/rake.rb:1112:in `verbose'
/Users/wsargent/.gem/ruby/1.8/gems/rspec-1.2.8/lib/spec/rake/spectask.rb:153:in `define'
/Library/Ruby/Gems/1.8/gems/rake-0.8.7/lib/rake.rb:636:in `call'
/Library/Ruby/Gems/1.8/gems/rake-0.8.7/lib/rake.rb:636:in `execute'
/Library/Ruby/Gems/1.8/gems/rake-0.8.7/lib/rake.rb:631:in `each'
/Library/Ruby/Gems/1.8/gems/rake-0.8.7/lib/rake.rb:631:in `execute'
/Library/Ruby/Gems/1.8/gems/rake-0.8.7/lib/rake.rb:597:in `invoke_with_call_chain'
/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/monitor.rb:242:in `synchronize'
/Library/Ruby/Gems/1.8/gems/rake-0.8.7/lib/rake.rb:590:in `invoke_with_call_chain'
/Library/Ruby/Gems/1.8/gems/rake-0.8.7/lib/rake.rb:583:in `invoke'
/Library/Ruby/Gems/1.8/gems/rake-0.8.7/lib/rake.rb:2051:in `invoke_task'
/Library/Ruby/Gems/1.8/gems/rake-0.8.7/lib/rake.rb:2029:in `top_level'
/Library/Ruby/Gems/1.8/gems/rake-0.8.7/lib/rake.rb:2029:in `each'
/Library/Ruby/Gems/1.8/gems/rake-0.8.7/lib/rake.rb:2029:in `top_level'
/Library/Ruby/Gems/1.8/gems/rake-0.8.7/lib/rake.rb:2068:in `standard_exception_handling'
/Library/Ruby/Gems/1.8/gems/rake-0.8.7/lib/rake.rb:2023:in `top_level'
/Library/Ruby/Gems/1.8/gems/rake-0.8.7/lib/rake.rb:2001:in `run'
/Library/Ruby/Gems/1.8/gems/rake-0.8.7/lib/rake.rb:2068:in `standard_exception_handling'
/Library/Ruby/Gems/1.8/gems/rake-0.8.7/lib/rake.rb:1998:in `run'
/Library/Ruby/Gems/1.8/gems/rake-0.8.7/bin/rake:31
/usr/bin/rake:19:in `load'
/usr/bin/rake:19