[ihaveajob]

Trello is beautiful, I'll give you that, but I miss having access to my data while offline. For that, I prefer Priority Matrix, which is as straightforward to use, and it allows me to work on a plane and sync when I get online.

[thedufer]

As mentioned elsewhere in the comments, the collaborative aspect means that allowing offline write access would end in non-trivial merge conflicts. It might be solvable, but due to the difficulty, it's definitely not a priority.