Document BuddyPress' BP-Default theme. Add PHP comments into the templates to help WordPress theme designers get started with BuddyPress. You can look at Twenty Ten's source code for an example of the type of thing we're after.
You'll need to have had some experience building WordPress themes, and an understanding of PHP. If you haven't used BuddyPress before, you'd need to be prepared to become familiar with the plugin.
When you claim a task, please include a short description of your experience with WordPress and any relevant information that will help us judge the likelihood that you will succeed at this task. Since only one student can claim a task at a time, if it doesn't seem like a good fit for your skills, we can recommend a different task.