Rails 2.1 - turing off time stamped migrations

I have read that you could turn of time stamped migrations by adding the line "config.active_record.timestamped_migrations = false" to your environment.rb. After I add this line I get the following error when running "script/generate model Post":

/Library/Ruby/Gems/1.8/gems/activerecord-2.1.0/lib/active_record/base.rb:1667:in `method_missing': undefined method `timestamped_migrations=' for ActiveRecord::Base:Class (NoMethodError)

Am I mistaken about this feature?

If I'm not mistaken about this feature has anyone else run into this? (I'd like to confirm before taking the next step.)

Thanks.
Mike

I found it detailed in this ticket http://dev.rubyonrails.org/ticket/11458
and here http://dev.rubyonrails.org/attachment/ticket/11458/timestamped-migrations.diff?format=txt
but I guess it didn't make it in to 2.1.