How to determine input device being used?

There are some input solutions where it would be really nice to know if a given input method is in use. Obviously if you know that you're on, say an iPad, you know the primary input is touch. If you are on a PC your primary input it likely to be mouse but with touch PCs coming out I need to know what input devices are really in use rather than making assumptions.

This is becoming even more acute on a web player. There are some situations where with a mouse I want to use absolute positioning but with touch I want to use relative positioning. I don't know if the user is interacting with touch or mouse (but I hope that there is a way to find out already in Unity).

Going beyond #if _UNITY_IPHONE etc. how can I find out what the input device actually is, (because going by what platform I'm on is an increasingly poor way to assume control method)?

Thanks.