There were 6 billion mobile subscriptions at the end of 2011

"The Measuring the Information Society 2012 report also identifies countries which have made the most progress when it comes to ICT development. These dynamic ICT markets are mostly located in the developing world – evidence that many developing countries are catching up quickly in efforts to bridge the so-called ‘digital divide’. Strong performers include Bahrain, Brazil, Ghana, Kenya, Rwanda and Saudi Arabia.
In the mobile sector, developing countries now account for the lion’s share of market growth. Mobile-cellular subscriptions registered continuous double-digit growth in developing country markets, for a global total of six billion mobile subscriptions by end 2011. Both China and India each account for around one billion subscriptions.
Mobile broadband continues to be the ICT service displaying the sharpest growth rates. Over the past year, growth in mobile-broadband services continued at 40% globally and 78% in developing countries. There are now twice as many mobile-broadband subscriptions as fixed-broadband subscriptions worldwide."
Source:  Press release from the ITU, 11th October 2012
Note - This does not mean that 6 billion people use mobile phones, as some reported.  Many people have more than one sim card (work phone, personal phone, iPad, etc)