It's a custom piece written by a guy called Toby Pitman, and hats off to him, I think it's a lovely piece of music which made me want to pick up the guitar again after a few years out!
I was searching for it myself, and after a bit of digging around, I can point you in the right direction:
http://web.mac.com/tobypitman/iWeb/tobypitmans _blog/Blog/Blog.html
You can download the soundtrack to the advert from there - it's 35 seconds long (hopefully he won't mind if it becomes popular, as it's on his own blog anyway - www.tobypitman.com):
I wanted to learn it on the guitar, so I can tell you the first part is something like Bm, FSm, G, A - and the chorusy bit begins with a D and bit of hammering on.
Hope that helps, and cheers to Toby if he ever reads this - It's an inspirational piece of music!
Stef