From Gartner’s 2010 report:
By 2013, mobile phones will overtake PCs as the most common Web access device worldwide. According to Gartner’s PC installed base forecast, the total number of PCs in use will reach 1.78 billion units in 2013. By 2013, the combined installed base of smartphones and browser-equipped enhanced phones will exceed 1.82 billion units and will be greater than the installed base for PCs thereafter.
Mobile Web users are typically prepared to make fewer clicks on a website than users accessing sites from a PC. Although a growing number of websites and Web-based applications offer support for small-form-factor mobile devices, many still do not. Websites not optimized for the smaller-screen formats will become a market barrier for their owners — much content and many sites will need to be reformatted/rebuilt.