What is a Javascript closure?

"a closure is a first-class function with free variables that are bound in the lexical environment"

"a closure is the local variables for a function - kept alive after the function has returned"

"when you pass a method reference out of the context in which it was defined, the method still has access to its parent context"

"a powerful, low level JavaScript library designed for building complex and scalable web applications"

Now the last one will be a bit harder to chew :)

Google released their internal JS library as Open Source code. And it looks sweet.

Until recently I was using Dojo for the better OOP architecture support as opposed to jQuery. Then I realized that the two serve different purposes. Now I use jQuery for augmenting my CakePHP views. It's much easier to do and you have a better chance to find a plugin to use as a base for your needed functionality.

But I wonder if Closure is better than Dojo?

From the first impression it looks that they drew a lot of inspiration from the Dojo Toolkit. Starting with the require/provide directives that instantly made me think "Dojo". I didn't get a chance to dive in yet and I'm still searching for some good online UI demos. I guess those will come soon but not from Google. I don't expect too much support from them and I suspect they'll leave the library follow it's own path once they released it into the wild.

Nevertheless Closure will be a tough player in the Javascript frameworks arena. If Gmail and Docs run on it then it's already production proven. But I guess jQuery is still better for what is normally needed in a web page. For one-page JS apps Dojo and now Chrome with their solid i18n, themes (Closure?) and templates support still rock.

and your routes shell be free

CakePHP routing and reverse routing does not work properly in shells. That is because your defined routes in routes.php are actually not loaded at all. Maybe 1.3 will fix this but until then read on. 
To have proper routes in the email messages you are sending from shells you need to do this in your shell at the top:

App::import('Core', 'Router');
include CONFIGS . 'routes.php';
define('FULL_BASE_URL', Configure::read('App.HttpHost'));

where App.HttpHost is something you set manually in your bootstrap because the shell doesn't have a clue about your site and the web server.
Bake on!