Obama has been running for president for two years, and some plumber asks a more digging question than any mainstream media reporter has asked the whole time. No wonder you got suspicious. After all, this is supposed to be a coronation, not an actual election.
Maybe after two years of Obama laying out his tax policy, reporters don't need to ask questions which have been answered hundreds of times.
Sen. Obama explained his reasoning for middle class tax cuts to Joe the plumber. The Senator spent five minutes answering Joe's question. And he has answered the question in every debate. And in many public speeches. Asking the same question one more time with partisan phrasing is not a "digging question."
A true "digging question" would be asking why John McCain feels the real "Joe the plumber" deserves to pay higher taxes.
I have a theory that it's impossible to prove anything, but I can't prove it.