Subversion 1.5 clients support relative urls:
See the Subversion 1.5 release notes and the subversion manual for more details
Some examples: (Note that the comment syntax is not supported)
# Repository root relative
^/projectX/trunk/src/ projectX
# Relative to current directory
../../dependency/X X
# Server root relative
/svn/repos/projectX/trunk/src projectX
# Schema relative
//svn.collab.net/repos/svn/trunk svn