Mac OS X Leopard’s subversion client (as of 10.5.6), or more accurately its subversion client’s dependency library libneon, appears to be slightly broken when used with Kerberised WebDAV.
As you’d expect, subversion 1.6 as provided by MacPorts does not have this flaw.
This blog entry (LI Tech Blog » Patching neon on OS X 10.5 for GSSAPI authenticated SubVersion) provides a solution to the problem. In summary: patch or replace libneon.
An interesting point is that the Mac build of the ‘pysvn’ subversion libraries does not appear to have been affected by this bug.