We have quite a few users with white space in their names and it would be awesome to being able to mention them as well.
The mention feature is so great to have so I'm thinking about asking everybody to modify their name if it is not possible to mention a name with white space.
I'm using Yammer at ABB and they have a drop down ala intellisense that displays matching names as you type. That makes it easier to type the name correct, and it works with white space. See that as an idea for improvement.