If you want to install that specific package you can just do
rpmbuild --rebuild subversion-1.6.11-7.el5.src.rpm
However the rpmforge repo has 1.6.6 in it, so you can enable that repo if you wish.
rpm -ivh http://packages.sw.be/rpmforge-release/rpmforge-release-0.5.2-2.el5.rf.x86_64.rpm
yum update subversion
 Assuming of course that x86_64 is your arch, if not you can change that to i386