I would really like Trello to support Undo. It would be great to have an Undo button and honour the OS keyboard shortcut also.
If you wanted to take it up a notch you could also follow a similar approach to the History in apps like Photoshop, by essentially adding journaling to Trello boards, and let the user go to Activity and click an "Undo to here" button to roll-back any changes up to that point.