Yes, I was going to ask this too. Their tour page says "Trello is free, now and forever.". Call me a skeptic, but I don't see that holding true unless they have some money to keep it running.
That would better be phrased as, "any features that are currently free, stay free." Of course we're working on adding paid features to Trello; see spolsky's replies elsewhere in this thread, or my comment history, for more details.