I love her boho style...love the bag too. Is it Marc Jacobs? It looks an awful lot like one that Cameron Diaz had (can't find the photo but hers was colourful - blues, reds, purple).
http://justjared.buzznet.com/2008/08/29/sienna-miller-baggy-jeans/#more-70401
http://justjared.buzznet.com/2008/08/29/sienna-miller-baggy-jeans/#more-70401