The beautiful folk piece in the Liberty Mutual commercial of July 4 is "Half an Acre" by Brooklyn folk group Hem.
MySpace link to entire song, streaming
ps I think it's funny that typing "half an acre" into google returns: half (1 acre) = 2 023.42821 m2
pps Hmm, googling "pi seconds in centuries" confirms that it is indeed a nanocentury
ppps Did I ruin the mood of that song yet?