The New York Times reports that the data for the Netflix Prize 2 will include more information about the anonymous users:
“Netflix was so pleased with the results of its first contest that it announced a second one on Monday. The new contest will present contestants with demographic and behavioral data, including renters’ ages, gender, ZIP codes, genre ratings and previously chosen movies — but not ratings. Contestants will then have to predict which movies those people will like.”
As others have noted this will make it much easier to “de-anonymize” individuals in the collection.
As an experiment, I checked the zip code where I grew up and found that it had about 3900 people in the 2000 census. So, given an age and gender you would have a set of about 40 people. With just a little bit of additional information, one could narrow this to a specific individual.
For example, Narayanan and Shmatikov showed (Robust De-anonymization of Large Sparse Datasets) that this could be done with the dataset from the first Netflix Grand Prize by mining information from IMDB. Think of how much more powerful such attacks would be with the new dataset.