I started out by switching everything over the nginx while keeping the mongrels alive, that was actually pretty easy.  Information was available everywhere.

Thinning everything via capistrano took a while, that wasn't as well documented.  Thin was documented, capistrano was documented, but easy solutions as to how to combine the two were difficult to find.

Here's the solution I was able to come up with -

Capistrano

My config for using mongrel used to look something like this -

[code lang="ruby"]
set :stages, %w(staging production)
set :default_stage, "production"

require "capistrano/ext/multistage"
require "mongrel_cluster/recipes"

set :application, "myapplication.com"
set :user, "appuser"set :repository,  "http://svn.myapplication.com/myapp/trunk"
set :deploy_to, "/var/www/#{application}"

role :app, application
role :web, application
role :db,  application, :primary => true

set :runner, user
set :keep_releases, 3
set(:mongrel_conf) { "#{current_path}/config/mongrel_cluster.yml" }

deploy.task :after_update_code, :roles => [:web] do
desc "Copying the right mongrel cluster config for the current stage environment."
run "cp -f #{release_path}/config/mongrel_#{stage}.yml #{release_path}/config/mongrel_cluster.yml"
end

...
[/code]

Now that we are moving from mongrel to thin, no need for two lines in particular, one being the line that requires mongrel_cluster recipes and the other that sets the mongrel_cluster yaml config path.  A third line changes from mongrel_cluster.yml to thin_cluster.yml.  You get something like this:

[code lang="ruby"]
set :stages, %w(staging production)
set :default_stage, "production"

require "capistrano/ext/multistage"

set :application, "myapplication.com"
set :user, "appuser"

set :repository,  "http://svn.myapplication.com/myapp/trunk"
set :deploy_to, "/var/www/#{application}"

role :app, application
role :web, application
role :db,  application, :primary => true

set :runner, user
set :keep_releases, 3

deploy.task :after_update_code, :roles => [:web] do
desc "Copying the right mongrel cluster config for the current stage environment."
run "cp -f #{release_path}/config/thin_#{stage}.yml #{release_path}/config/thin_cluster.yml"
end
...
[/code]

Now we need to implement what mongrel recipes was doing for us, start, stop and restart but in terms of thin (added this to the bottom of my deploy.rb):

[code lang="ruby"]
namespace :deploy do
desc "Restart the Thin processes on the app server."
task :restart do
run "thin restart -C #{release_path}/config/thin_cluster.yml"
end
desc "Start the Thin processes on the app server."
task :start do
run "thin start -C #{release_path}/config/thin_cluster.yml"
end
desc "Stop the Thin processes on the app server."
task :stop do
run "thin stop -C #{release_path}/config/thin_cluster.yml"
end
end
[/code]

Here's what my thin_cluster.yml looks like:

[code lang="ruby"]

---
log: log/thin.log
address: 127.0.0.1
port: 9000
chdir: /var/www/myapp.com/current
environment: production
pid: tmp/pids/thin.pid
user: www-user
group: www-data
servers: 3
[/code]

That's it and it has worked out nicely so far.