If you have a project of any significance for which you are using or considering using Ruby on Rails, you will have to justify your decision to several brutal audiences. Your customers, investors, those that may acquire your business, your coworkers, managers, team, etc. have a whole boatload of concerns about technologies that are not regarded as mainstream. If you're not using something widely regarded as safe (J2EE or .Net), you'd better be prepared to dance.
Performance and Scalability
The top two concerns are usually performance and scalability: "Ruby is slow" or "Interpreted languages are slow" seem to be the most common. Here are some excellent articles on the subject (the overall summary being: Rails provides a great advantage in getting your product to market, there are reasonable and inexpensive ways of addressing any performance bottlenecks, and scalability is not a problem):
It's Boring to Scale With Ruby on Rails
Scalability Examples: Hardware Requirements at Basecamp and Robot Co-Op (43Things)
Outsourcing the Performance-Intensive Functions
Making Things Faster
Large Systems Using Rails
There are a lot more than listed here, but these are some of the largest:
Amazon Uses Rails on Amazon Unspun
Basecamp has over 1 million users
43Things has over 700,000 users
Odeo is a Rails App
Blinksale is a Rails App
List of Other Rails Deployments
Philosophy Behind Rails
Many of the arguments against Rails are rooted in a misunderstanding of philosophy, purpose, and design decisions behind the framework. The following articles provide clarity in this area:
Ruby on Rails: An Interview with DHH
IndicThreads.com interview with DHH
The Reg sits down with DHH
Choose a Single Layer of Cleverness
Are You Afraid?
Fear-Driven Technology Choices
Is Rails Ready for Prime Time?
Are We Approaching a Tipping Point for Rails?
Will Rails Become Mainstream?
Does it Matter If It Does?
How to Introduce Ruby on Rails in Your Company
Paul Graham: Beating the Averages (It's about Lisp as a strategic advantage. Just read "Rails" where it says "Lisp" and you'll get the idea).
TIOBE Programming Community Index One indicator of the rate of Ruby popularity (as of this writing #12 and moving up fast).
Some of these articles are dated and are not up to date with respect to capabilities of the current version of Rails, but nevertheless provide good material.
RoR in the Enterprise Toolbox
Rails / J2EE Comparison from IBM
Rails Bidding: Put Your Money Where Your Mouth Is
Rails Perspective from the guy at the bank
Know the Enemy
Get to know what the other side is saying. There is a lot of writing out there (some rational and well reasoned, some misinformed, and some from people that are almost certainly insane). Search for 'rails sucks' or 'ruby sucks' and you'll have hours of entertaining reading. You need to understand what Rails strengths are, what its weaknesses are, and what FUD is out there in order to be able to make (and defend) a wise decision.