Trello works in offline mode. Just create cards or edit existing ones and it gets uploaded when you have an Internet connection. Tried it today and it just works.
Oh, is this a recent change? The last time I tried, it had a message along the lines of 'connection down, changes you make now may not be saved'
And then some of the changes were not saved, I tested.
Trello does not support offline operation. It may work sometimes but offline sync is not a feature of any of the official clients.
Correction: on review this is not the case as none of the changes I made were actually permanently saved to the cards. Sorry about the incorrect information.