Google Experiments with Search Within Search

Google has begun offering secondary search boxes within their search results (above). Typical of Google offerings, not everyone is seeing the "feature" yet, as it rolls out to different data centers.

The secondary box restricts the search to the site in question. It seems like the searches that are most likely to bring up the secondary box involves retailers (as above), although The New York Times brings up a secondary box and some say YouTube and Wikipedia do also (though I did not see that result).

How are these sites are chosen? Do they have to pay for this extra search box, or is some algorithm at work?

Since it's not available in all data centers yet, if you really want to see it now, you can try this data center where the new feature is working.

What do you readers think of this new feature? Try it out and let me know.