But that music stuck with me, so I decided to track it down. Thankfully, AdTunes.com came through again: Chopin, Frédéric. 24 Préludes, Op. 28, no. 15. Prelude in D-flat Major ("Raindrop").
Here's a YouTube video of someone playing the full song. There isn't anything to see--unless you really like watching six minutes of nothing but a tight shot on the hands of someone playing piano--so open it in another window and let the music play in the background while you go about your websurfing.
The section they used for the commercial starts at about the 2:00 mark and runs through about 4:30 or so.
Feudalism: Serf & Turf
P.S. For any piano players out there, scans of various 19th century publications of Chopin's scores are freely available from the University of Chicago's Chopin Early Editions website. In this version from 1839, the section used for the commercial starts at the bottom of the first page and runs through the middle of the third page.