I created a forum posting that discussed this: http://forums.atlassian.com/message.jspa?messageID=257356762.

Our build environment consists of six different SVN repositories that branch at different times in our release cycle. We have multiple plans that build from the same repository so we define our SVN URLs as global variables. As we branch and create new plans, it would be ideal if the global SVN variables could be updated/defined via the REST API. This is currently a manual task.