I keep hearing about Rails being used by startups but have not heard of, say, Morgan Stanley using it. Is there a list somewhere of bigger companies that use Rails internally?
Twitter is the most widely used rails implementation I know of.
There's a nice list here: http://rubyonrails.org/applications
EDIT: MTV and Oracle seem to be the "biggest" companies in that list.
Enterprise's generally only use proven technology with a long track record (jsp and asp.net), especially for anything mission critical. I don't think rails fits those criteria yet.
My most recent employer (one of the major defense contractors) was a fortune ~50 corporation and I was actively developing with Rails to build internal applications.
You can also take a look at this list from Obie Fernandez on big companies working with Rails.
http://blog.obiefernandez.com/content/2008/03/big-name-compan.html
I dunno about Morgan Stanley, but Rails is offered as a platform for intranet development within Deutsche Bank, as a lightweight alternative to more "traditional" Java-based solutions.
I think the reasons are fairly obvious: robust, well-maintained framework, concise & powerful language, speed of implementation, cost (or lack of) and let's not forget the fun factor - even bankers like to have fun...
According to the attachment to Message/Job Board in RailsConf, Twitter is still using Rails and in need of corresponding developers: