Whom or who, again?

"Number two: the 'who' or 'whom' business. This is very slightly trickier. 'John, whom I know to be an honourable man' is right; 'John, whom I know is an honourable man' is wrong. Here's what you do: you mentally recast the subclauses as main clauses — 'I know him to be an honourable man', 'I know he is an honourable man' — and your ear will guide you: 'him' demands 'whom', and 'he' demands 'who' . . . In conversational prose be wary of whom. In the closing pages of Herzog, Bellow writes, 'Whom was I kidding?' This is grammatically correct; it also leaves the sentence up on one stilt. 'Whom the fuck d'you think you're looking at?' Or even worse, 'At whom the fuck d'you think you're looking?' Never worry about ending a sentence with a preposition. 'That rule', Churchill famously said, 'is the kind of pedantry up with which I will not put.'"

~~ from Inside Story: A Novel by Martin Amis