Here's the steps I took to create this error:
rvm use ruby-1.9.2-p180@sporkfail --create
gem install bundler
gem install rails --pre
rails new sporkfail
cd sporkfail
# edit Gemfile
bundle install
rails generate cucumber:install --spork
rails generate cucumber:feature post title:string body:text published:boolean
rails generate scaffold post title:string body:text published:boolean
rake db:migrate
bundle exec rake cucumber
# works
bundle exec spork
# error
Same thing happened to me when I upgraded to 1.0.