Technically speaking, on an IFR flight if ATC asks you to stand by for your clearance, are you still VFR and therefore responsible for your own flight clear of obstacles?
Does this then imply that one cannot commence such an IFR flight in conditions below VFR minima?
Correct.
In the UK, Class G, an instrument qualified pilot can enter IMC, because IFR in Class G does not require radio contact.
Outside the UK, this is a grey area because there is no possibility of a clearance in Class G anyway.
I am assuming this relates to the activation of an IFR flightplan in the air as opposed to prior to departure?
"We" don't know what flight plan this pilot filed, and those who know are not saying. I assume he had a JAA IR (ME) if he requested an IFR clearance.
It is certainly easier to elevate a flight plan to IFR if you have filed a Z flight plan (which specifies a transition to IFR at a specific waypoint) otherwise ATC have to knock up a routing for you. In the southern UK, this is almost impossible but French ATC can do it pretty quickly (minutes, IME).