In Rails 1.1+ (apparently, this has been around since at least May 2006), there's been an extension added to Object class, and that's to_json(). That's right. You can serialize any object in rails to JSON, simply by calling to_json(). However, it's not documented in the Rails API, so your best bet is to google for it.
As a quick and dirty way to demonstrate it, you can have this in your controller:
# some type of data from somewhere in the database
@data = Data.find(1)
and then the following in your view somewhere:
var data = <%= @data.to_json %>
<%= act_on_data(@data) %>
Neat. So in this way, you can pass your data from your server to code in your views that isn't to be displayed in HTML, but in some other form. One person has tied Adobe's Flex in with Rails for this very purpose. Tip!
But one other thing that I realized after reading through JSON for the masses, is somewhat startling...at least to me. And in hindsight, perhaps it was because it never seemed easy to do in XML.
This kind of reminds me of how Von Neumann's computer architecture was novel at the time because it treated program commands and data as the same thing. Before, computers had one place for data, and one place for a program. You didn't store programs the same place that you stored data. By the same token, it also reminds me of how Lisp's data is pretty much its code, and vice versa, and that's where it derives a lot of its power.