From Google's recommendations for URL structure:

Sorting parameters. Some large shopping sites provide multiple ways to sort the same items, resulting in a much greater number of URLs. For example:

http://www.example.com/results?search_type=search_videos&search_query=tpb&search_sort=relevance&search_category=25"

When linking from outside, then having URLs differing only by sort parameters is obviously a bad idea: Google will not understand that these links point to the same item, i.e. that the item is popular. Therefore ranking will be lower than it should.

But what's the alternative? Using a fragment identifier (#), and then doing the sorting in JavaScript? What else? Some settings in Webmaster tools?