There is an interesting tidbit at the beginning of a Search Engine Land post, "Kicking The Tires On Shopping Search, Part Two: The Independents".
Compiling data from Hitwise, they shows that Google, Microsoft, and Yahoo now have a meager 15% market share in shopping search combined, down from nearly 50% combined three years ago.
Considering that a substantial percentage of web search queries are shopping-related (see "A taxonomy of web search" (PDF)) and the ease of extracting advertising revenue and revenue sharing where there is such strong purchase intent, I would think that Google, Yahoo, and Microsoft would be pursuing shopping metasearch more aggressively.
See also part one of the Search Engine Land series, "Analyzing the Major Shopping Search Services", which focuses mostly on design and usability of the shopping sites offered by Google, Yahoo, and Microsoft.
See also my earlier posts, "R.I.P. Froogle?" and "What should Google do next?".