Some other people are already doing this. I had the vague idea for a while, but only bothered to actually do anything about it after reading Katja Grace's post about Beeminder on OB. The relevant part is this:

• points accrued for doing tasks on my to-do list. When I think of anything I want to do I put it on the list, whether it’s watching a certain movie or figuring out how to make the to do list system better. Some things stay there permanently, e.g. laundry. I assign each task a number of points, which goes up every Sunday if it’s still on the list. I have to get 15 points per day or I lose.

I think getting too bogged-down in the details of the system could be harmful in the long run. So far I've got a TODO.txt with a table of tasks I need to do regularly, a table of tasks I need to do just once or very occasionally, and a table for each day where I list the tasks and tally the points. I'll revise the points as I go, but the hard part for me is to get into the habit of using the list.