After a few weeks, at our company we've developed a scrum plugin integrated with Mantis

Some of the features:
- Create / Manage Sprint
- Move tasks from backlog or between sprints
- Burnt-down chart
- Quick add issues for sprint
- In-line edit on sprint page to assign user
- Print sprint to html
- Reports per sprint (task allocation and resource usage)
- Track estimated time vs real time
- Timesheet for daily updates
- Track tasks worked across sprints
- Export tasks with hours worked to CSV
Screenshots coming shortly
We do not have any plans to release it to the public yet, but if you are interested, please PM.
Thanks,