What options does Terminal pass to bash on startup? In the Terminal preferences, I'm able to configure the Terminal to "Open shells with:" -> "Command (complete path):" /opt/local/bin/bash.

What options is the terminal passing when invoking this command?  Terminal appears to run each shell as a login shell, so I'm assuming --login is being passed?  

Is there any Apple documentation that gives an overview of exactly what is being run when you open a terminal window?
The command:

/opt/local/bin/bash


essentially instructs Terminal.app to load an interactive bash shell (the path for your example is likely the macports version). Initially when you open Terminal it starts a login shell session (in bash). Login shells can read files such as ~/.profile, ~/.bash_profile that setup environment variables for your session, so what is being run is mostly indicative of what is configured within those files. 

In general, by default when opening a bash shell from Terminal these things would be run:

Processes

bash
login


Open Files and Ports

/bin/bash
/usr/lib/dyld
/private/var/db/dyld/dyld_shared_cache_x86_64
/dev/ttys000 


More Information:


  Official Bash Website
  
  Apple Man Page: bash(1)