Bought X but not Y and other sub-query filters in reports

I'm not sure how urgent this is, but it would be nice to be able to do reports of contacts who have open opportunities, Contacts who have no opportunities, and you can come up with loads of similar examples (divisions with no contact with the "main contact" rolecode, etc).

Beyond that, it would be nice to be able to do Customers who have bought X product, or who have not bought Y product.

The pinnacle is "customers who have bough X but not Y".