Exactly - how can a bunch of RoR people be guilty of "price fixing" when a customer is free to employ PHP, .NET, J2EE, ... people to do exactly the same job? It's not like we have a monopoly on Web development.
Now, if you're talking about locking in existing RoR customers, for whom it may be prohibitively expensive to rebuild their working RoR apps in e.g. J2EE, you might have an argument. Even then, there's nothing preventing those customers from finding someone else and having that person learn RoR as part of the assignment - it's not like us RoR guys hold the only magic keys to the kingdom.
In general, I give existing customers a *lower* rate, on the basis that I know there won't be a problem getting paid, that the working conditions are a known quantity, that they won't be looking over my shoulder constantly questioning what I'm doing, that there's that much less of a chance of nasty surprises, etc.
Regards
Dave M.