It would appear to be true (and Bing doesn’t deny it):
Google has run a sting operation that it says proves Bing has been watching what people search for on Google, the sites they select from Google’s results, then uses that information to improve Bing’s own search listings. Bing doesn’t deny this.As a result of the apparent monitoring, Bing’s relevancy is potentially improving (or getting worse) on the back of Google’s own work. Google likens it to the digital equivalent of Bing leaning over during an exam and copying off of Google’s test.
Yikes! Do you feel burned? Or should you even care (or have an opinion)?