The Subversion bug database contains many issues classified as "bite-sized" tasks -- tasks that are well-defined and self-contained, and thus suitable for a volunteer looking to get involved with the project. You don't need broad or detailed knowledge of Subversion's design to take on one of these, just a pretty good idea of how things generally work, and familiarity with the coding guidelines in the HACKING file. Each task is something a volunteer could pick off in a spare evening or two.
If you start one of these, please notify the other developers by mailing dev@subversion.tigris.org (of course, you should be subscribed to that list). If you have questions as you go, ask the dev list, and send the patch there when ready.