I just noticed today that a search on Amazon (e.g., this search for algorithms) does not provide the options to sort the results or to refine by anything other than category. Once you do select a category (e.g., books), you are given additional refinement options, as well as the ability to sort.
While I find this interface less than ideal (e.g. even if all of your search are in a single category, it still makes you select that category explicitly), I do commend them for recognizing the need to have users clarify before they refine. The implication–one we’ve been pursuing at Endeca–is that it is incumbent on the system to detect when its understanding of the user’s intent is ambiguous enough to require a clarification dialogue.
The Noisy Channel 