I've encountered a number of incidents where images I have "shared" with other VPhot users have gone missing from both my and their accounts. It doesn't happen all the time, only occasionally. Doc, Geir and I are currently troubleshooting it by attempting to narrow down the circumstances around when files go missing. If anyone experiences something similar, please post a reply here describing in as much detail as you can the files that were missing, who you shared them with, when you shared them and when you noticed them missing (including time), etc.
Thanks.