I disagreed with this statement because there are plenty of successful sites that has other dots besides .com.  It's all depend on your application.  If your application is useful, then people will come.  For example, when I registered my website, <a href="http://www.mathmaster.org" rel="nofollow">http://www.mathmaster.org</a>, the .dot com and everything else were taken but now I am still doing ok in term of visitor counts.  Majority of my users come from either bookmarked or search engine anyway.