This is awesome news for a variety of reasons. First of all it's great for development (you can take a look at the github repo here: https://github.com/sagemath/sage. There's a good talk about the git workflow for development by +Volker Braun here: http://www.youtube.com/watch?v=0tejiKN5ctY.
The other great reason why this is awesome is that it just got really easy to use and install Sage.
Here's a short video demonstrating everything I've done below:
If you're familiar with git then you know this but if you're not then you can simply open up a terminal on anything *nix (linux/Mac OS) and type the following:
$ cd ~
$ git clone git://github.com/sagemath/sage.git
This basically goes to the git repository on github and clones it to a folder called sage in your home directory (if you don't have git installed you'll have to do that first).
Once you've done that you need to 'make' sage:
$ cd ~/sage
This will take a little while (it goes and gets most of what you need so it's hard to say how long as it depends on your machine) but after that you'll have Sage on your machine. If you're still in the ~/sage directory you can simply type ./sage to start sage.
You'll want to add sage to your path so that you can use it from any directory. In this video I did this by using a bit of a trick but here's I'll do something simpler: create a symbolic link to the sage file in ~/sage directory and place that symbolic link in your path (in /usr/bin/local). To do that type this:
$ ln -s ~/sage/sage /usr/local/bin/sage
Now you can type sage anywhere and you'll get sage up and running.
What's really great about all this is that if and when updates/development happens you can just git pull to get all up to date changes. Based on the +Sage Mathematical Software System post on G+: here: it looks like you can already play around with the develop branch...
Of course if you want the easiest way to use Sage then simply grab an account on +The Sagemath Cloud. I gave a talk last week at the Cardiff Django/Python user group about it and +William Stein was kind enough to drop in and take some questions: http://www.youtube.com/watch?v=OYVLoTL4xt8 (sound quality isn't always great because I move around a fair bit...)