Not only is Google announcing all sorts of stuff today, but it's also updating a slew of its apps (it is Update Wednesday, after all). Today's Google Search update brings a huge improvement: "OK Google" hotword detection to any screen... even the lockscreen. If you're plugged into a charger, the feature will even work with the display off. This lines up with the earlier rumor that it was coming eventually.
Once enabled (Google Now > Settings > Voice > "Ok Google Detection"), you'll have to enable Audio History and say the words three time in order for Now to register your voice – much like setting up the hotword detection on the Moto X and recent Droid devices.