undefined method `parse_query_parameters' for ActionController::CgiRequest:Class

I've just deployed an update to one of my sites, and now I get this
error in the production log. The update was just a simple update to
one of the erb templates, adding a link to the sites blog.

I've tried rolling the site back to the previous version, but the
error still persists, it's all very frustrating.

The full error report is pasted below.

/!\ FAILSAFE /!\ Mon Mar 24 09:50:23 +0000 2008
  Status: 500 Internal Server Error
  undefined method `parse_query_parameters' for
ActionController::CgiRequest:Class
    /usr/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/
cgi_process.rb:74:in `query_parameters'
    /usr/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/
request.rb:287:in `parameters'
    /usr/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/
session_management.rb:128:in `set_session_options_without_components'
    /usr/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/
components.rb:157:in `set_session_options'
    /usr/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/
session_management.rb:122:in `process'
    /usr/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/
base.rb:388:in `process'
    /usr/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/
dispatcher.rb:171:in `handle_request'
    /usr/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/
dispatcher.rb:115:in `dispatch'
    /usr/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/
dispatcher.rb:126:in `dispatch_cgi'
    /usr/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/
dispatcher.rb:9:in `dispatch'
    /usr/lib/ruby/gems/1.8/gems/mongrel-1.1.3/bin/../lib/mongrel/
rails.rb:76:in `process'
    /usr/lib/ruby/gems/1.8/gems/mongrel-1.1.3/bin/../lib/mongrel/
rails.rb:74:in `synchronize'
    /usr/lib/ruby/gems/1.8/gems/mongrel-1.1.3/bin/../lib/mongrel/
rails.rb:74:in `process'
    /usr/lib/ruby/gems/1.8/gems/mongrel-1.1.3/bin/../lib/mongrel.rb:
159:in `process_client'
    /usr/lib/ruby/gems/1.8/gems/mongrel-1.1.3/bin/../lib/mongrel.rb:
158:in `each'
    /usr/lib/ruby/gems/1.8/gems/mongrel-1.1.3/bin/../lib/mongrel.rb:
158:in `process_client'
    /usr/lib/ruby/gems/1.8/gems/mongrel-1.1.3/bin/../lib/mongrel.rb:
285:in `run'
    /usr/lib/ruby/gems/1.8/gems/mongrel-1.1.3/bin/../lib/mongrel.rb:
285:in `initialize'
    /usr/lib/ruby/gems/1.8/gems/mongrel-1.1.3/bin/../lib/mongrel.rb:
285:in `new'
    /usr/lib/ruby/gems/1.8/gems/mongrel-1.1.3/bin/../lib/mongrel.rb:
285:in `run'
    /usr/lib/ruby/gems/1.8/gems/mongrel-1.1.3/bin/../lib/mongrel.rb:
268:in `initialize'
    /usr/lib/ruby/gems/1.8/gems/mongrel-1.1.3/bin/../lib/mongrel.rb:
268:in `new'
    /usr/lib/ruby/gems/1.8/gems/mongrel-1.1.3/bin/../lib/mongrel.rb:
268:in `run'
    /usr/lib/ruby/gems/1.8/gems/mongrel-1.1.3/bin/../lib/mongrel/
configurator.rb:282:in `run'
    /usr/lib/ruby/gems/1.8/gems/mongrel-1.1.3/bin/../lib/mongrel/
configurator.rb:281:in `each'
    /usr/lib/ruby/gems/1.8/gems/mongrel-1.1.3/bin/../lib/mongrel/
configurator.rb:281:in `run'
    /usr/lib/ruby/gems/1.8/gems/mongrel-1.1.3/bin/mongrel_rails:128:in
`run'
    /usr/lib/ruby/gems/1.8/gems/mongrel-1.1.3/bin/../lib/mongrel/
command.rb:212:in `run'
    /usr/lib/ruby/gems/1.8/gems/mongrel-1.1.3/bin/mongrel_rails:281