Arguably, negative equity only affects you if you want or have to move, so not everyone who is in the shit as a private home owner is going to be "f***ed" as Simster puts it. But those who had multiple houses as investments are going to be far more hit.
by that reckoning, Buy-to-letters would not be affected either then, so long as their prpoerties continue to be let at rates which cover the loan repayments.