Multiple customers - keeping the data separate - how?

I'm trying to get a handle on the different ways of maintaining data
separation in Rails. From what I've read it looks like usually the
security is handled entirely as an aspect within the Model.

I constantly find it amusing that whenever a 'new' way of doing
applications is created, they always ignore the security systems that
have gone before. First we had operating system security with its user
and group database. Then we have databases with their own security
model. Now we have web apps reinventing that particular wheel once
again sitting in a single operating system user space and logging onto
the database with a single all powerful user.

Unfortunately the application I have in mind involves account data, and
I can't afford a bug in an application exposing one customer's data to
another. I need something more substantial than that. (And there are
other reasons - such as backup). However I still want to share physical
infrastructure.

My thoughts are that there should be a URL per customer driving their
own mongrels locked onto their own version of the database. However the
standard infrastructure support tools don't support that way of doing
things.

Are there any other thoughts about how the security separation should
be enforced?

Rgs

NeilW