Pontius & NuName,
Back in the day when I was a young F/O and keen to ensure that my logbook was filled out completely correctly, I put the 'Instrument flight time' question to my Base Captain, he referred me to my logbook instructions page, and CAA regulations, which stated that instrument flight time is "Time during which a pilot is controlling an aircraft in flight solely by reference to instruments."
Now, where I disagree with you both, although I stand to be corrected by a valid point of reference, is that IFR is relevant to this. NuName, when you're tootling along at FL300-FL400, IMC or VMC has no relevance as you're almost certainly in IFR airspace and navigating by reference to the instrumentation in front of you. That is clearly instrument time as defined by the above guidelines. If en-route you're cleared to a certain waypoint, you don't turn towards the waypoint using a visual reference, but again using the instrumentation in front of you. IMC alone does not determine instrument time.
As I personally operate mostly in IFR airspace, as soon as the wheels come up I'm navigating on instruments, even if the autopilot is in and coupled I'm still monitoring it. The same is the case at the other end (unless cleared for a visual approach) with the approach being on instruments too, including radar vectors.