Why does that matter? As long as they didn't borrow more than they could afford to repay per month (in which case they deserve to get slung out on the street), then negative equity isn't anything new.
It doesn't really mean anything - even if these people need to move house, then the property that they will be moving into will have dropped by the same amount too. As long as people can afford to carry on paying their monthly payments, it's all just numbers on paper.
The reason people complain about negative equity and dropping house prices is because they somehow feel they have a "right" to automatically move up the property ladder without actually doing anything. As mentioned before, these people just have to accept that buying a house is an investment in property, and investment values can, indeed, "go up or down".
I am part of this too: the value of our house is in freefall at the moment, but so are all the houses around us, so if we really want to move, it can still be done...... just because it looks like turning out now that we would have been better waiting for house prices to come down before getting on the property ladder, that's our mistake - we, just like the millions of others threatened with negative equity, are perfectly capable of dealing with it. It was an investment mistake - that's all - get over it!