AV8R and Skymonkey,
It does not matter if you have half the wing of your Navajo or 182 in cloud, you are not VMC so by definition you are IMC but you cannot log I.F. as you can still see external features (ground, horizon, the pyramid, sugar cane, Mount Bartle Frere etc).
When you can no longer see any things outside, to reference the aircraft by visually, then it's IF.