Fact: Come Rails 3 time there will be no reason to use any other Web Famework for Web 2.0 Applications.
Ruby 1.9 will be out and will fix performance issues. NO REASON TO USE JAVA ANYMORE.
Merb will merge with Rails and Rails will finally scale. NO REASON TO USE JAVA OR PHP ANYMORE.
Rails 3 will be completely component-ized and modular. Granular! Take what you need, leave the rest. NO REASON TO USE PHP ANYMORE.
That's right. Your favourite web programming language will die. There are no exceptions. Ruby is better than all.