It's easy to write an application that won't scale using pretty much any technology. In particular, the more complex the configuration and layering, the more opportunity (IMO) for a tight coupling that won't scale out well. Alternatively, you can use a shared-nothing architecture and scaling out becomes a far easier task. Many enterprises don't need scaling in the sense of handling millions of pageviews a day so be sure you have a problem before you identify scalability as a criterion.
In terms of development team structure, this will take some getting used to. ThoughtWorks is putting teams onto enterprise Rails projects to good effect, so they might be a good consulting resource if you get hung up on this. If I were setting up a team, I would make the cornerstones of development:
1. Spec first, code later. That way it doesn't so much matter who does the implementation so long as the behavior is described accurately
2. Create business-relevant user stories.
3. Identify thin vertical slices and implement these in your iterations.
4. Only use developers who are comfortable with distributed source-code control (like Subversion).
Rails is pretty darn opinionated about what parts of the application logic belong where. You can make changes to suit your needs, but at the end of the day, what's in Rails, and really fire-tested is the three-tier model-view-controller pattern where the model handles both database communication and business logic, the controller handles glue between model(s) and views, and the views are the presentation layer. You can add layers (like presenters) but right out of the box, the separation of responsibilities is pretty easy to grasp and new developers brought onto the team who have Rails experience will understand your code pretty quickly.
I don't know anything about your business needs, but if you have any doubts about Rails' ability to do the job, find a consultant with a portfolio of clients similar in size to your company. Hire them to write a whitepaper on the pros and cons of using Rails to solve a particular problem (including specifics of traffic, security requirements, time-to-develop, maintenance costs, labor pool, etc.). Getting a professional with this kind of experience to provide you a technology assessment will provide a recommendation from an expert with known qualifications and very likely would begin a business relationship between you and that company as you bring your team online.
I mentioned ThoughtWorks because they have a good client portfolio. There are a number of other, smaller companies who could be even better positioned to provide the information you need.
This would be my reply in general to anyone who is interested in Rails for your enterprise application. Hire a pro. Pay that person or small team well to spearhead your effort.
