There was an academic paper on this published in the fall by Telefonica in Spain: http://conferences.sigcomm.org/hotnets/2012/papers/hotnets12...

Abstract:

  Price discrimination, setting the price of a given product
  for each customer individually according to his valuation
  for it, can benefit from extensive information collected
  online on the customers and thus contribute to the
  profitability of e-commerce services. Another way to
  discriminate among customers with different willingness to
  pay is to steer them towards different sets of products
  when they search within a product category (i.e., search
  discrimination). Our main contribution in this paper is to
  empirically demonstrate the existence of signs of both
  price and search discrimination on the Internet, and to
  uncover the information vectors used to facilitate them.
  Supported by our findings, we outline the design of a
  large-scale, distributed watchdog system that allows users
  to detect discriminatory practices.

The paper only has preliminary results (hotnets targets fairly early ideas), but so far has only detected discrimination based on location and search terms.