When you start a new Rails project using the rails generator, it creates a shebang line in dispatch.fcgi specific to where ruby is installed. For example, on FreeBSD, the line is
#!/usr/local/bin/ruby18
On Mac OS X, the line is
#!/opt/local/bin/ruby
I use FreeBSD in my production environment and OS X in my development environment. Instead of maintaining two different dispatch.fcgi files or adding extra logic to the files, you can easily solve this problem by making a symlink in the development environment.
I first changed the line to the appropriate one for my production environment, and then created the following symlink.
sudo ln /usr/local/bin/ruby18 /opt/local/bin/ruby
This is probably more of an issue for people on vastly different platforms. However, since FreeBSD and OS X are so similar, I might just standardize on /usr/local/bin/ruby.