You can fly in IMC conditions all day long in class G airspace. You need a clearance to enter controlled airspace. That's why when you depart a uncontrolled field with class G airspace, your clearance will state "upon entering controlled airspace, ATC clears XXXXX to do something.

The key issue with Admin V. Murphy was that the pilot had no information as to if VMC conditions would be present when the plane reached controlled airspace and the pilots statments that he was going to be operating under VFR when the conditions were clearly not VMC.

As stated by others, the key is untedstanding the the difference between IFR (that's the regulations) and IMC (that's the weather).