The more usual word for someone whose net worth is negative, measured by the whole of their debts and assets, is "bankrupt".

That's not the word either. Obviously simple 'debt' isn't the word: someone with a million dollars in cash who owes his mate ten bucks for the meal the other night. But 'bankrupt' means a different thing again. If you have a $1m mortgage on a house and the property prices have fallen then the value of your assets may well have fallen below that needed to cover your debt but you still aren't bankrupt. At least, not yet and not while you can keep up the payments. Unless your lawyers and accountants recommend it as an option.

I would have guessed the word for what Hugh was referring to would be "net debt" but that is a bit off too (since it doesn't take into account long term assets, just liquid assets). Just plain "deficit net worth" is the simplest description I know of but it seems to be something that deserves a word of its own. Anyone know of one?