I just received this as well with foreman 0.3. My error was slightly different:

Processing EnvironmentsController#destroy (for ###.###.###.### at 2011-06-09 14:24:21) [DELETE]
Parameters: {"authenticity_token"=>"YIGfIErFy9v+4nsVZFnmFGRrzwRIO48dQ0MpCEanwtg=", "id"=>"beta"}
You may not destroy beta as it is in use!

NameError (undefined local variable or method `load_vars_for_ajax' for #<EnvironmentsController:0x7fe87306af00>):
app/controllers/environments_controller.rb:51:in `destroy'
passenger (3.0.2) lib/phusion_passenger/rack/request_handler.rb:96:in `process_request'
passenger (3.0.2) lib/phusion_passenger/abstract_request_handler.rb:513:in `accept_and_process_next_request'
passenger (3.0.2) lib/phusion_passenger/abstract_request_handler.rb:274:in `main_loop'
passenger (3.0.2) lib/phusion_passenger/classic_rails/application_spawner.rb:321:in `start_request_handler'
passenger (3.0.2) lib/phusion_passenger/classic_rails/application_spawner.rb:275:in `send'
passenger (3.0.2) lib/phusion_passenger/classic_rails/application_spawner.rb:275:in `handle_spawn_application'
passenger (3.0.2) lib/phusion_passenger/utils.rb:479:in `safe_fork'
passenger (3.0.2) lib/phusion_passenger/classic_rails/application_spawner.rb:270:in `handle_spawn_application'
passenger (3.0.2) lib/phusion_passenger/abstract_server.rb:357:in `__send__'
passenger (3.0.2) lib/phusion_passenger/abstract_server.rb:357:in `server_main_loop'
passenger (3.0.2) lib/phusion_passenger/abstract_server.rb:206:in `start_synchronously'
passenger (3.0.2) lib/phusion_passenger/abstract_server.rb:180:in `start'
passenger (3.0.2) lib/phusion_passenger/classic_rails/application_spawner.rb:149:in `start'
passenger (3.0.2) lib/phusion_passenger/spawn_manager.rb:219:in `spawn_rails_application'
passenger (3.0.2) lib/phusion_passenger/abstract_server_collection.rb:132:in `lookup_or_add'
passenger (3.0.2) lib/phusion_passenger/spawn_manager.rb:214:in `spawn_rails_application'
passenger (3.0.2) lib/phusion_passenger/abstract_server_collection.rb:82:in `synchronize'
passenger (3.0.2) lib/phusion_passenger/abstract_server_collection.rb:79:in `synchronize'
passenger (3.0.2) lib/phusion_passenger/spawn_manager.rb:213:in `spawn_rails_application'
passenger (3.0.2) lib/phusion_passenger/spawn_manager.rb:132:in `spawn_application'
passenger (3.0.2) lib/phusion_passenger/spawn_manager.rb:275:in `handle_spawn_application'
passenger (3.0.2) lib/phusion_passenger/abstract_server.rb:357:in `__send__'
passenger (3.0.2) lib/phusion_passenger/abstract_server.rb:357:in `server_main_loop'
passenger (3.0.2) lib/phusion_passenger/abstract_server.rb:206:in `start_synchronously'
passenger (3.0.2) helper-scripts/passenger-spawn-server:99

Rendering /srv/www/usps-puppet-foreman/public/500.html (500 Internal Server Error)