[nhayden]

The case for infringement would be very weak to non-existent if they recreated the look and feel writing the CSS themselves from scratch. The issue is they took the original product and modified some spacing and other minor details and redistributed it. Any written work like this is copyrightable.

[nickysielicki]

You're forgetting that they wrote the entire backend as a part of this.

[bdcravens]

Quickly reading through the code, it looks like the frontend is the bulk of the code. Which makes sense: Trello is really a pretty simple app; the UI experience is where the real magic is.

[bbcbasic]

Could even be done as a couchapp.