Making the mousewheel available through love.mouse.getWheel or love.mouse.isDown.
The mousewheel is currently the only input device that requires use of a callback to get its information. Every other input device has a getter method to return the condition of the device.
SDL 1.2 handles the mousewheel as a button so it would be consistent, if a bit illogical, to expose the mousewheel as a button through love.mouse.isDown for each frame the event is triggered in addition to the callback.
An alternative to exposing it through isDown would be to have a method to return the mousewheel state as "up", "down", or "center" like half of a joystick hat would help deal with the time between now and SDL 1.3/2.