Millions to miss out on the net. Is it pithy of me to point out that it depends what you mean by "online"?
If you mean "connected to a network", then I think mobile phone penetration stands at just shy of 80% in the UK. Yes: that's not web access, or broadband, or any of the lovely things which the digital media industry spurts over. But even low-bandwidth connectivity broken into 160-character chunks can be used to accomplish a lot.
...which is not to suggest that there isn't a digital divide, or that getting broadband into the homes of the nation isn't a worthy idea. Just that this stuff gets really interesting not when it can shove 16 million pixels in your face, but rather when it's so weaved into the fabric of our everyday lives that we don't notice it - and mobile is at this point.