Does anyone knows if there is any kind of interface builder to visually build your web app with ruby on rails 3?
I don't know if it still does, but the Mac application Flux from http://theescapers.com had this ability in version 2. It was one of their big selling points. That said, a good CSS editor is pretty much all you need, and one that can do live reflection, like CSSEdit or Coda, can allow you to do nearly WYSIWYG changes to your layout. As long as your basic generated HTML is set up to allow good "hooks" for styling, you should have no trouble making all sorts of visual changes.
How do I integrate it with Ruby on Rails?
That Flux program makes a new project, how can I use buttons and stuff at my TextMate project for rails?
Should I make the holy website and then copy and past the css and html ?
I would point both applications at the same folder, and let Flux make the view stuff and Textmate make all the rest.
So first I start the project with rails generate and then edit what i need (like controllers) from textmate and views