Is there a way to move single cards between boards? I have lists that grow into becoming a project of their own and would be more managable in a separate board.
I've just tried having two boards open at the same time, in two different Firefox windows, and attempting to drag a card from one board to another. You can even drag the card out of the first window, so I believe that the answer to your question is "No".
However, the idea is being considered, and you could vote for it in the Trello on Trello board. I see it's in the "Next Up" list, which sounds promising!
Trello tweeted yesterday (2011-11-17) that this now implemented:
NEW! Move cards between boards. Just click the 'Move' button, select a different board, and sit back in amazement.