Different configuration for dev/test/prod

I'm trying to figure out the best approach to having different
configurations for when I'm deploying to a development machine (just a
single mongrel running from a directory in my home directory), a production
machine (service script via sudo to start mongrel cluster, deployed to a
directory under /var/www, maybe multiple machines), or a test machine (same
as prod, but a different machine).

I don't think roles are the right approach. I could have different config
files and load the right file based on a -S variable or with a -F flag, but
that's kind of ugly.

What I'd like is to have a dev, test, and prod namespace in which all of
the deploy namespace is included, with overrides as necessary. Then I'd cap
dev:deploy or cap test:deploy:restart or the like. Is there a way to import
one namespace into another?

--Greg