I believe the large success that Ruby has enjoyed has made programmers eager to find the next big language (NBL). The NBL holds a lot of promise for the aspiring software developer. Become an expert in the NBL and you'll be able to command a premium while riding the next wave. If your lucky you may also design the testing or build framework that will ensure stardom in the NBL community.
So, how can you be ahead of the curve? How can you be one of the first to master the NBL? Tim O'Reilly already told us how to start: watch the Alpha Geeks.
The NBL might be a language that already exists. Dave Thomas recently blogged a bit about Erlang. Avi Bryant continues to invest his time in Smalltalk. And, it would be foolish to ignore Paul Graham and the Lisp community.
Then again, the next big language might be Ruby with YARV, Rubinius, JRuby, XRuby, or IronRuby. It could also be a language that exists but has a small community such as IO or Boo. Or, it could be a new language with more or less rope.
I think part of the problem with guessing what the next big language will be is that the question assumes there will be one language that can satisfy all your needs. The truth is, differing systems have very different needs. How many web applications have you worked on that actually needed an object oriented language? Because of the request/response nature of the Internet, most web applications could easily be built using a procedural language. There's tons of PHP sites that work just fine without objects. Sure, some sites are complicated and need a bit more than PHP4. For example, building Yahoo Store must require an Object Oriented language, right? While Yahoo Store was probably a bit complicated for PHP4, Paul Graham lists Lisp as their secret weapon of success. I'm not implying that OO languages are better than Lisp or vice versa. I'm simply pointing out that a language choice is probably not something worth making at an Enterprise level. The same is true when looking for the NBL, you aren't likely to find one that is perfect for solving all possible problems.
So then the question becomes, of the various NBLs of the future, which one is designed to address the problems you are interested in? If multi-processors and concurrency interest you, Erlang might be the ticket. If you love building web applications, Ruby/Rails isn't likely to go away any time soon. Or, if you want to build on years of experience with the same language, Smalltalk and Lisp have plenty of lessons for you to learn.
In my opinion it doesn't matter what NBL is. Most languages have unique solutions that allow you see problems in an entirely new way. That's why the Pragmatic Programmers suggest that you learn a new language every year. So instead of looking for the NBL, learn a language with a paradigm that differs from the one you use at your day job. For example, if you are currently using an OO language, learn a functional language. The established languages of today have plenty of lessons for you to learn. Knowledge of Ruby, Smalltalk, C#, Java, Lisp, Erlang, PHP, Perl, etc will ensure that you will quickly become proficient in any language you adopt in the future.
If you still insist on trying to predict the future, here's something to consider. As an industry we spent many years learning the lessons of procedural languages. Next, the majority learned the lessons of statically typed object oriented languages. Now, more than ever, we are learning the lessons of dynamically typed object oriented languages. If I had to guess, I think sooner or later the masses are going to figure out that functional languages also have power to offer.