Now that so many people are using mobile devices to browse websites, there is a much greater need for sites that are optimized for the mobile web.

There are a number of ways to incorporate a mobile site, ranging from automatic generation of mobile pages under the same URL's as the desktop version of the website to having dedicated mobile pages to even having special (sub)domains for mobile devices.

Using the same URL for both the mobile and desktop version has huge benefits. The biggest benefit is that for users the URL does not matter: people might enter a URL by hand, are originating from a RSS feed or follow a link from another website and are expecting specific content. 

Many would argue that redirecting the user based on a browser is the wrong approach. They would argue that a webpage should always be aware of the capabilities/limitations of the browser and adapt accordingly, regardless of the type or age of the browser. But you might choose to do this anyway. In fact there is a very good case for presenting totally different content based on the browser: one of the most important arguments is that the information needs of mobile users might be totally different.

Adding a specific subdomain (mobile is most commonly used, resulting in mobile.yourcompany.com) to your site is another way  of opening your website for mobile users. This could be very useful if you would deliver other content to mobile users than you would for desktop users.

Another option is to use the .mobi top level domain for your mobile version (resulting in www.yourcompany.mobi). Disadvantage of the .mobi domain is that it has another top-level domain, basically requiring you to redirect users.

The last two solutions have the disadvantage that you have to direct users to use the proper entrance of your site, but they are still viable options.

The final determination on how to incorporate a mobile version of the website is based on many factors and needs to be handled on a case-by-case basis.