Paul just announced that Subversion repository of PostGIS source code has moved to OSGeo server. The new URL of SVN is: http://svn.osgeo.org/postgis/.
Only base location of the repository has changed, so the cheapest way to switch existing working copy that points to old location of SVN is to switch & relocate:
mloskot@dog:~/dev/postgis/_svn/trunk$ svn switch --relocate \
http://svn.refractions.net/postgis/trunk/ \
https://svn.osgeo.org/postgis/trunk
For the sake of security, I would recommend committers to use https scheme which is well supported on OSGeo server.
