The display of websites are typically controlled by stylesheets. Stylesheets tell the browsers how and where to display elements on a page. To create a mobile site it is simply a case of creating a new stylesheet for delivery on mobile platforms.
Most functionality is also available on mobile platforms so customers will be able to search for products and even complete transactions. Granted this is an untested technology but the barriers to entry are extremely low. Furthermore the major search engines have all created indexes specifically aimed at mobile content so it is clear that they at least think this is going to take off.
Going mobile of course has some design constraints. With the screen size being considerably less, information architecture and usability become even more important. This makes building a successful mobile site harder as you have less space to work with. But for businesses with a mature and established website this is certainly something they should at least consider.
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