How can I restore/repair my broken /etc/sudoers file?

You probably want to ask that question on the Unix forum, but here's what i think you will have to do:
[Learn about vim|http://vim.sourceforge.net> (it's the text editor that you'll be using to change the file).
[Enable the root user and log in as root|http://support.apple.com/kb/ht1528].
Open Terminal and run visudo
That may be enough to reset the permissions so it works and all the text is still there. Otherwise, you'll have to add this:

# sudoers file.
#
# This file MUST be edited with the 'visudo' command as root.
# Failure to use 'visudo' may result in syntax or file permission errors
# that prevent sudo from running.
#
# See the sudoers man page for the details on how to write a sudoers file.
#
# Host alias specification
# User alias specification
# Cmnd alias specification
# Defaults specification
Defaults env_reset
Defaults env_keep += "BLOCKSIZE"
Defaults env_keep += "COLORFGBG COLORTERM"
Defaults env_keep += "_CF_USER_TEXTENCODING"
Defaults env_keep += "CHARSET LANG LANGUAGE LC_ALL LC_COLLATE LC_CTYPE"
Defaults env_keep += "LC_MESSAGES LC_MONETARY LC_NUMERIC LC_TIME"
Defaults env_keep += "LINES COLUMNS"
Defaults env_keep += "LSCOLORS"
Defaults env_keep += "SSHAUTHSOCK"
Defaults env_keep += "TZ"
Defaults env_keep += "DISPLAY XAUTHORIZATION XAUTHORITY"
Defaults env_keep += "EDITOR VISUAL"
# Runas alias specification
# User privilege specification
root ALL=(ALL) ALL
%admin ALL=(ALL) ALL
# Uncomment to allow people in group wheel to run all commands
# %wheel ALL=(ALL) ALL
# Same thing without a password
# %wheel ALL=(ALL) NOPASSWD: ALL
# Samples
# %users ALL=/sbin/mount /cdrom,/sbin/umount /cdrom
# %users localhost=/sbin/shutdown -h now

Log out of root and disable the root user (see above link).

Yeah, I don't know how to use vim, either. That's why you need to read up on it.
[Apparently, you need to use :w|http://vimdoc.sourceforge.net/htmldoc/editing.html#:write]

Jannis, once you’ve logged in as root and opened Terminal, I’d recommend the following steps:

• Enter *cd /etc* and *ls -l sudoers* — in your case, the information shown might be something like

-r--r----- 1 root wheel 0 Mar 30 01:23 sudoers

The first field is a representation of the file’s permissions. The default permissions for the file (shown above) are “read-only access for the root user and all users in the wheel group; no access for anyone else”. The fifth field (the lone zero above) is the size of the file.

• Run the visudo command. When vim is running, press the lower case letter a to start adding text, copy and paste the text that Barney-15E provided, and press the Escape key to stop adding. Press :w to write the text to the sudoers file, and press :q to quit vim. (Alternatively, press either :wq or ZZ (lower-case in the former, upper-case in the latter) to write the file and quit vim in one command.)

• Run the *openssl dgst -sha1 -c sudoers* command on the restored sudoers file to make sure that it has the identical “signature” as its original contents. If it has an exact copy of its original contents, the following output should be returned:

SHA1(sudoers)= 81:0f:97:11:83:7c:80:49:7f:56:89:d5:cd:50:34:4b:81:6c:e4:29

• Ensure that the default permissions, owner, and group of the sudoers file are in place by the following commands: *chmod ug=r,o= sudoers* and *chown root:wheel sudoers*. Running *ls -l sudoers* should then show something like

-r--r----- 1 root wheel 1242 Mar 31 12:34 sudoers

• Now that you have a restored original sudoers file, save a copy of it so that if future circumstances require it, you can restore it easily. Something like *cp -p sudoers sudoers.original* will make it easy to find in the /etc directory.

Jannis, I’m glad that I could help. I have no idea if the same standard sudoers content is provided on every Mac OS X install, but I’m not aware of any reason why different content would be needed on different installations; as long as every Mac OS X system has a root user and an admin group, the same default sudoers file should work equally well on all of them.