In this episode Alex Centeno MBA. from Merkados, shows you how to install the necessary code to retrieve the IP addresses of your visitors using Google Analytics.
Also, you could use this method to generate custom segmentation other than by IP address. For example, you could create the segment by browser, by keyword (the keyword that sent the visitor to your site) or any other collected information: such as email address or name. The possibility are truly endless.
** Remember that this technique populates all profiles in your Google Analytics account and there is no way to remove this information from your account once it is there. So make sure you want it there.