Salesforce1 is indeed built on Aura, as confirmed in Josh KaplansKaplan's (Salesforce Aura Product Manager), DreamfroceDreamforce 2013 session, Introducing Aura: Rapid Mobile Development with Reusable Components. It is also using the Salesforce Mobile SDK as well. This great collection of Dreamforce 2013 sessions featuring Salesforce1 also references the Aura session. Salesforce drinking its own champagne!
In that presentation btw, Josh and his team give some great insight into how production grade Aura really is, the journey they have taken with it since Salesforce Touch (which also used an early build) and a sneak peak at where it is going as native platform feature! I personally cannot wait, one to watch!
The source code is shared in GitHub, so you can even try it out and contribute!