The sendmail m4 config files are located in /usr/share/sendmail/conf.  
Everything you need to reconfig your sendmail from the m4 files 
should be there.  The default sendmail.cf as shipped on your machine
was generated from the /usr/share/sendmail/conf/cf/generic-darwin.mc
config file.  If you wish to make changes, the following steps are
recommended:
* cd to /usr/share/sendmail/conf/cf:
	cd /usr/share/sendmail/conf/cf
* Copy the default config file to yourdomain.mc:
	sudo cp generic-darwin.mc yourdomain.mc
* Edit your new config file:
	sudo vi yourdomain.mc
* Regenerate your sendmail.cf file from the m4 file you just edited:
	m4 ../m4/cf.m4 yourdomain.mc > /tmp/sendmail.cf
* Backup your old sendmail.cf:
	sudo cp /etc/mail/sendmail.cf /etc/mail/sendmail.cf.orig
* Put your new sendmail.cf in place:
	sudo cp /tmp/sendmail.cf /etc/mail/
* Restart sendmail

Other oddities of using sendmail on Mac OS X:
* aliases are first taken from netinfo, and then sendmail falls back on
  the aliases file.
* The aliases file lives in /etc/aliases although no default aliases file
  is shipped in the stock Mac OS X config.  You can change the location
  of your aliases file by using the following line in your sendmail m4
  config file:
  define(`ALIAS_FILE', `path/to/aliases')
  Or you can change it in your sendmail.cf file with the following line
  (probably commented out in your config):
  O AliasFile=/path/to/aliases
* To ensure that sendmail does not read its config file from netinfo, 
  execute the following commands:
niutil -create . /locations/sendmail
niutil -createprop . /locations/sendmail sendmail.cf /etc/mail/sendmail.cf 
  This will tell sendmail (when it looks at netinfo) to not look in
  netinfo for its config file, but to look at /etc/mail/sendmail.cf