Canonical's new tablet operating system Ubuntu on Tablets includes Voice Control in the HUD.

From Ted Gould's HUD 2.0 page:

With the HUD we realized that we had a relatively small data set, and so it would be possible to get reasonable voice recognition using the resources available in the device. [...]

We built the voice feature around two different Open Source voice engines: Pocket Sphinx and Julius. While we started with Pocket Sphinx we weren't entirely happy with it's performance, and found Julius to start faster and provide better results. Unfortunately Julius is licensed with the 4-clause BSD license, putting it in multiverse and making it so that we can't link to it in the Ubuntu archive version of HUD.

Link to the source: http://bazaar.launchpad.net/~indicator-applet-developers/hud/phablet/files/head:/src/

Edit: I should mention that they are working with the CMU sphinx group to resolve the Pocket Sphinx performance issues they experienced - likely just a configuration issue.