Are there any open-source worm filesystems?

I recently came across Nexenta’s WORM plugin for their commercial storage system, which is based on the OpenSolaris kernel and ZFS. For those of you that aren’t familiar with the term, WORM means ‘write-once read-many’ — in other words, after it’s written it cannot be modified, but can be read any number of times. I’d love to see support for this in an open-source filesystem (specifically, I’d like to store my photo archive on WORM, where once I’ve copied the photos off the flash disk any updates/edits need to go to a parallel read/write folder), but am not finding anything – if anyone is aware of a project that implements this, I’d love to hear about it!