well for db:migrate modifying ENV works very well, but if you want to
pass a certain RAILS_ENV it seems to be impossible from inside an
active rails-app. you'll have to go with system calls.

  system('rake db:drop RAILS_ENV=test')

but

  ENV['VERSION']= '0'
  Rake::Task['db:migrate'].invoke

so far even on tha rake mailing-list no other option occurred.