Trello can be quite a useful task-tracker, but it's got the usual pitfall of being a cloud service that might change at any time and become unusable (expensive or stupid or whatever).
Luckily, it's a simple matter to set up a daily cronjob to run Matthieu Aubry's trello-backup script. It produces a single JSON file for each board.
Now, restoring from that file might be a different matter...