This is pretty new and I like it. Google's now letting you use Google Now as an alert service.
You can see the the "Keep me updated" button in the screen grab below. I triggered it via a Voice search but have re-created with a text search for [Who is David Tennant].
If you click the button Google replies by saying;
You will get updates about David Tennant in your Google Now cards
Google Now tends to be associated with mobile devices. This is one of the first explicit desktop to mobile command you can do (although there are plenty of implicit ways to connect your Google Now with your desktop searching).
As of yet I've not received any David Tennant Google Now alerts but should I do I'll try and take a screengrab.
Anyone able to re-create? Or has this been around for ages and I've simply not noticed?