I have blogged a few times about digital portfolios, particularly in the early years. The reasons for portfolios that we have explored are to replace paper portfolios, assessment for reporting, evidence for parent meetings, to monitor improvement etc. One reason we are yet to explore is for students to engage in self assessment and reflection. Older students could definitely be given the responsibility of maintaining their own digital portfolios.
This is something you would need to have a play with in your own class to discover the method that would suit you best. As we have, generally, one iPad per three students at KPS I would envision three students having a portfolio in Book Creator on each iPad. Obviously there would need to be strict rules about not going into other people's portfolios. To help students organise their information effectively and to ensure that reflection takes place, a template should be developed. This template should give students guidance but still allow them to reveal their own creativity and personality. In portfolios students can include digital work samples from the camera roll, photos of non digital work, videos of activities such as listening and speaking activities, in app drawings, audio or text reflections, screen shots of task descriptions and/or rubrics.
At the end of each term or the end of the year the portfolios can be kept by the student in the form of an ePub.