Nice idea - and also something similar for uploads. If I've kicked of a large download from someone and I leave my computer switched on overnight, chances are that the upload will finish long before I next return to my machine. I'm a broadband internet user and so a fair bit of bandwith has been wasted. Why not free up all available bandwith for uploads while my machine is idle?

And as stated above, dynamic allocation of connections according to available bandwith is a v. good idea - in both directions.