I know I've had this problem before so I'm really frustrated.

I've got the Ruby RSpec bundle installed for TextMate, but when I Run Behaviour Description or Run Focused Example I get this wonderful error:

/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/rubygems.rb:827:
in `report_activate_error': Could not find RubyGem rspec (>= 1.1.0) (Gem::LoadError)
from /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/rubygems.rb:261:
in `activate' from /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby
/1.8/rubygems.rb:68:in `gem' from /Users/simon/Library/Application Support/TextMate/Bundles/Ruby
RSpec.tmbundle/Support/lib/spec_mate.rb:13 from /tmp/temp_textmate.oWRPUR:3:in `require'
from /tmp/temp_textmate.oWRPUR:3

(I added linebreaks to make it readable)

I'm using macports so my rspec gem is installed in /opt/local/lib/ruby/gems/1.8/gems/. Why isn't it finding it?

In Preferences > Advanced > Shell Variables my TM_RUBY is set to /opt/local/bin/ruby. I also tried the trick here:

http://dnite.org/2007/8/28/textmate-and-your-environment-variables/

... which didn't do anything.