It looks like Google isn't done adding goodies to the latest round of updates for the official Search app. The hotword for activating the voice search function is now "Ok Google," and it also works while looking at results, not just from the standard search screen. You'll need Search 2.8.7 (grab the APK here if you don't have it yet) and an updated English (US) language pack to see the new functionality.
Just when you thought we were done with the already rather extensive Google Search teardown, another wild teardown appears. Yup, still the same good old Search 2.7, but this time, we found a hidden feature that you will really want, even more than custom hotwords. At least I think you will.
Turns out, there's a secret flag within Search that lets you use the hotword, set by default to 'Google,' anywhere in search results instead of just the home Activity.