Use of subdomains for a family of related sites is a perfectly reasonable, non-abusive thing to do; I do this with such sites as webtips.dan.info and mailformat.dan.info (as subdomains of dan.info), and a whole lot of other sites do it too: Wikipedia has its different language sites as subdomains of wikipedia.org; craigslist has various city subdomains; and, of course, Google does many of its services using subdomains of google.com (maps.google.com, etc.). It's more logical than using Stupid Unnecessary Domain Names for everything, registering lots of names with your base name as a substring (which, unfortunately, Google also does a lot of).