It seems to me that Folder sharing / collaboration isn't properly supported. Correct me if I am wrong, but the only way to access someone else's tasks is through the web site, right?
There is absolutely no support for sharing tasks in any other interface to the Toodledo data (so I assume the API just doesn't support it). At the end of the day, this means I can't access shared tasks on my iPhone, my iPad, or anywhere else. I can't have alerts from shared tasks, I can't email shared tasks to myself. In summary, I cannot use shared tasks in any kind of practical way.
As far as I can tell, this limitation is not explicitly made clear anywhere. In your documentation about sharing/collaboration you do not say anywhere: Note: sharing is only supported through the web interface.
I would love to stand corrected...if not, I think you need to make the limitations clearer, or better still, roll out the functionality to the API so that it becomes properly usable.