I'm not sure why you would want to. My philosophy has always been that having one website in an area is much more effect then having several. The reason for this is if you concentrate all your link building time on one site, instead of spreading the time around you end up with one site that can achieve more rankings then a handful of other sites.
That being said there are some nefarious tactics I can think of with having multiple domains but there grayish hat and not something I'd disclose on the forums
