Is there some technical reason I'm missing for why they wouldn't go the Passenger route? Seems to be by-far the easiest way to deploy a Rack-capable web app, and it (correct me if I'm wrong) allows for "hot deploy": update the code on the server and it'll sit there unused until you touch "tmp/restart.txt" under your app root.
I had issues with passenger at an event where a couple thousand people were relying on my app (about 1 thousand concurrent). I had a single dual quad machine running a rails 3-pre app on ruby 1.9, which was mem leaking every 30-45 min and melting down the goods. On site, I switched from nginx + passenger to apache + passenger so I could use a max-requests-per-worker type directive (only available with apache + nginx), which didn't solve my problems. I made one more leap (on the spot) to nginx + unicorn, and problem solved. It also saved me at least 1gb of normal operational ram.
I still think passenger is great for low demand sites. It took a while to figure out everything that had to be done for unicorn, which included a proper unicorn config file, a rake task to start/stop/restart, and a matching init.d script for my ubuntu; I also wrote a rake task to install the init.d script. Someone should post that stuff online.
