Hi Iohannes,
another important issue is, that svn knows nothing about priviliges (you cannot restrict users to read-only in a special branch (or rather subdirectory, as that is how svn handles branches) directly. such privilege separation is currently used in the pd-CVS on sf. i am pretty sure that it is possible by sophisticated usage of the apache-<Limit>, but i have no experience on making it work together smoothly.
i don't think this is true any longer for the current svn series 1.3. It includes the mod_authz_svn module which can be used for path-based authorization.
greetings, Thomas