Regarding existing holders, if you have your own keys you are relatively ok (minus the obvious destruction of USD value), but the situation with coins in online exchanges and wallets will be "problematic" if say an exchange with 500k BTCs, say 'ok my clients, now you have 500k BTCCs because we adopted this fork' (and we are keeping 500k BTCs of the other fork for ourselves). It would be like stealing BTCs and exchanging them for Gavincoins.
This is a legit concern. However, there will be plenty of notice. Anyone who is genuinely worried about this should ensure their Bitcoins are withdrawn to keys they hold themselves, as always (and as you say. Missed that bit).