This is what I'd like to see as well. I am in a board of a foundation and with the four of us we have a shared list. We use this shared list to remind eachother of waiting tasks, and to enable our secretary to enter the Action Points from meetings for all of us.
Currently we use tags to note who is first responsable, but it would be nice to connect a task with a contact. Connecting tasks and contacts would also enable the development to:
- show only your own tasks in the current and/or other (smart) lists (depending on your choice for that shared list).
- get personalised statistical overviews (over time, compared with other team members, etc.)

i agree: would be great if each task could include an associated contact or at least a meta-field for contact info (e.g. phone-number for the restaurant or doctor, etc.)