I finally found out how to enter special characters the right way in
Carbon Emacs.  Previously, when I hit “ä” and the likes, it would tell
me about “Undefined keys” and the like.  Interestingly, the character
C-q ä on my german keyboard creates a totally different character
than it’s supposed to be (0x8a vs 0x8e4).  I helped myself adding
these characters with TextEdit afterwise, well knowing that this
approach can’t be the whole truth.
However, this is now fixed. I simply added
(set-keyboard-coding-system 'mac-roman)
to my .emacs and now I can enter the correct characters with a
single keypress.
I still haven’t found out how to make Emacs actually display these
characters, though: I still only see empty boxes.  Not that bad,
but not perfect either.  Comments on this would be very welcome.
Update 30jan2005: I fixed that issue!  The code you need in your
.emacs is:
(create-fontset-from-fontset-spec
 "-apple-monaco-medium-r-normal--9-*-*-*-*-*-fontset-mac,
  ascii:-apple-monaco-medium-r-normal--9-*-*-*-m-*-mac-roman,
  latin-iso8859-1:-apple-monaco-medium-r-normal--9-*-*-*-m-*-mac-roman,
  mule-unicode-0100-24ff:-apple-monaco-medium-r-normal--9-*-*-*-m-*-mac-roman")
(set-frame-font "fontset-mac" 'keep)
(add-to-list 'default-frame-alist
         '(font . "fontset-mac"))
NP: Interpol—Not Even Jail