I'd presume that Apple will integrate a Siri-like interface to the next OSX, in say the 2012/2013 region, so that you can do basic things like say "subscribe to the NPR politics podcast" and "play the NPR politics podcast". This could also have a keyboard interface like Quicksilver and perhaps even hooks for an Applescript user community (but Apple doesn't really do community). Ubuntu on the other hand has the very good Gnome-Do and the impressive Synapse[1]. However, Ubuntu doesn't have a speech recognition engine to even begin to match Siri so you'd be stuck with whatever Google evolves on top of voice in HTML5/Chrome. That would be limited to website interactions and perhaps Google apps (i.e. when in an email, only in Gmail, about a conference perhaps you could say "add this to my calendar", only to Google calendar).

One feasible path of exploration would be to use CMU Sphinx[2] connected to local NLP and Unity (or some underlying infrastructure that KDE and Gnome can share). But even if Shuttleworth hired a couple of developers now and had them getting some traction in 2012, you'd be looking at a usable beta for Ubuntu 13.04. At that point We'd be looking at the iPhone 6+ and one or two iterations of Android on top of ICS.

Microsoft has the base technology lying around in terms of speech recognition and Bing/MS Research/Kinect team and could probably come out with an answer on the next iteration of Windows (desktop and mobile). So you will have a first class solution on all Apple products, and a solid implementation on Windows as well as Android and perhaps Chrome. Nevertheless this forces Canonical's hand and they more or less have to move on this or they will be left behind.

My suggestion would be to take the following strategy (if Shuttleworth is reading): hire two or three developers or so for integrating Sphinx as a core service, and hire two or three developers to leverage NLTK on top of python (already the Ubuntu system language) and connecting it to Synapse. That's just four to six developers and two apt-gets being default rather than optional. Further, to save money, I'd contact a number of universities in the UK (Canonical is in London, logistics and legal are thereby simplified) to sponsor a few Masters and PhDs on basic research in the area with the proviso that their research goes open source and is patent free. In this way, a developer salary for one year turns into three years of research from a PhD. As I understand it they already have some people working on Kinect-like projects, but it needs far more resources otherwise it's just left to Google, who more or less don't care about the Linux desktop and tie the Android stuff to the cloud. Otherwise Ubuntu with a few tweaks is going to get slaughtered (if there is even one percent of market share to actually slaughter) by a cohort of competitors looking like you'd imagine OSX"11"/iPhone 6 and Kinect 2/Xbox 4 to be. The only other option would be to rely on the Google API from a native app, which would mean you could probably deliver something in Ubuntu 12.10, but you'd be totally beholden to Google then: and that's only if they wanted to let you use their servers with no ads going the other way.

[1]http://www.omgubuntu.co.uk/2010/11/synapse-gnome-do-launcher...
[2]http://cmusphinx.sourceforge.net/