The challenge you might run into is that ROR developers charge a lot more than the PHP or even .NET developers. Its still young, "hip" technology and because ROR developers are in higher demand you run into higher contract rates.
@david is right that you want to sell your experience in creating apps. Most of your clients will not know the difference if something is coded in Perl, CGI, PHP, Cold Fusion, ASP, or ROR. To them a site is just a site. So, you need to sell the the benefits of ROR, letting them know that its MVC pattern makes sure that updates can be done quicker, and performance might be better versus languages that allow for Spaghetti coded techniques.
Sell yourself as a firm, Sell your dedication to service, quality and performance. Tell your clients that you have a vested interest in their success, and that you understand that if you build a web app or site you expect it to outshine the competition. Last, you could offer more channels to attract customers such as Graphic Design, Print Design, and SEO, funneling them all towards your bread and butter, (development)
Best of luck.