Mobile ads are a tiny fraction of Google's business.
The future of small, handheld computing devices is coming.
Desktop computers will remain popular for work in information based economies like ours, but there are billions of people out there who will access the Internet soley from mobile devices.
(I'm currently sitting in front of my computer but am writing this on a mobile device.)
It's not whether or not you are tethered to a desk, but what OS you run and what your relationship to your computer is. I have been using notebooks for the past 5 years and I would not call me mobile in any but the most literal sense.
Mobile advertising is growing, but that doesn't make it a big part.