to_json in Rails 2.0.2 not generating proper json?

I am using Rails 2.0.2 and trying to use to_json to generate json. I
notice several problems.

1) datetime field such as created_at mapped to "created_at": {}
2) :except does not work, @post.to_json(:except =>
[:created_at, :updated_at] still gives me "created_at": {},
"update_at": {}
3) problem with escaped char mapping: '<' mapped to '\074' instead of
'\u003C'

Is there problem with my installation? Is there a module I should turn
on?
Do I need jsonifier plugin?

Help please!
victor