Is now possible.
Well, sort of. Late yesterday, Koushik Dutta pushed out this post on Google+:
Koush explains how to make this happen, and there is 1 caveat: you must use a rooted Nexus 5. Obviously, Koush is working to make mirroring available for more devices.
Finally, I have a reason to root my device.
Another Cyanogenmod developer also showed what’s possible with mirroring in this post:
I’m pretty sure that’s above most people’s understanding, but this simply means there are additional possibilities for using Koush’s Mirror app.
So, if you’re the owner of a Nexus 5, and you’d like to root your device, Mirror app is available. Hopefully, in the not-so-distant future — Koush is regularly updating his Chromecast apps — Koush will push Mirror (without root) to the masses.
Note: I couldn’t find Mirror when browsing Google Play on my Nexus 5, but it’s available using my desktop.