Check out this infoQ bit where Martin Fowler, of ThoughtWorks, talks about using Ruby/Rails for three years - and their experiences.
To answer your question, I'm not sure that we'll see another single language dominate the way COBOL, Java, or .net have. I think we are in a new age of Polyglot programming where multiple languages will be the norm in 5-10 years. (JMHO)
I see that Ruby is also a testbed for ideas that are then adopted by other languages, and I think it'll keep that role for the time being. Apart from that, I don't dare to predict the future 5 years from now... The Internet has a longer memory than that.
Paolo Perrotta, Europe
Author of Metaprogramming Ruby