Chris Sherman has a nice overview of new shopping engines and their differentiating features. Most noteworthy from my point of view is Become.com , which offers a "research" button and a "shop" button. (Click the links in the previous sentence to see the differences in the results for "digital camera.") Yahoo! (and others) have experimented with similar strategies.
There's considerable skepticism about consumers' understanding of and willingness to use such tools, but I think they offer intriguing possibilities for delivering more relevant results—and "disambiguating" consumer intent—at each "stage" of the buying cycle. I wrote more about that in a previous post (scroll for discussion).
I think there are several (not necessarily linear) stages of the purchase cycle that could be addressed in this way: general research (identifying the product/service), editorial/user reviews and product comparisons (features), price comparisons, and where to buy (online/off). This approach creates distinct ad inventory at each stage: graphical branding ads at the research stage, PPC (and coupons) at the price comparison stage and maybe pay-per-phone call at the buy (locally) stage, for example.
Shopping/comparison engines will continue to gain momentum over time as e-commerce grows. But again, look for more "where can I buy it locally?" offerings on the main comparison engines going forward.