I found this really good article (and subsequent thread) about SVN "branching for feature" and merging back to the trunk for release. We have a trunk which is always the next patch version of the current released version. But we also branch the tagged current release for long running development on future features. This article describes the issues with merging trunk work into these development branches, and then merging the development branches back to the trunk.
Subversion merge reintegrate by Mark Phippard, Director Subversion Engineering at CollabNet.