Apache & SVN on Ubuntu - Post-commit hook fails silently, pre-commit hook “Permission Denied”
So, after discussing this issue with a very helpful system administrator at the BEST hosting provider ever, RailsPlayground, they were able to fix the problem by setting the permissions on /dev/null to
crw-rw-rw- 1 root root 1, 3
and now, all SVN hooks work as expected.