Despite a number of challenges and set-backs, my school (grades K-3) is now using Posthaven as an ePortfolio. Posthaven is created by a couple of the creators of Posterous, which no longer exists. My school used Posterous as an ePortfolio platform last year.
Posthaven is similar, although not exactly the same, as Posterous.
Each teacher has a Posthaven account. Within that account we created a Site for each student. Whenever a student creates work on the iPad they email it it their Site. Each Site has a specific email address associated with it. Posthaven looks like a blog--each piece of work shows up in chronological order.
Posthaven can accept anything that can be emailed--links, text, photos, PDF, video, etc. This is a big advantage of Posthaven compared to some of the other options we looked at. As a teacher I can comment on student work with either a grade or text.
Parents have access to their child's Site, but otherwise the ePortfolio is private. The work in the ePortfolio can be transferred to next year's teacher so the ePortfolio will grow and "travel" with the student.