For a beneficiary (merchant/exchange/whatever), the safest thing to do in case of a fork would be to require that transactions clear in both blockchains until it is clear which blockchain wins; However, this gets a bit difficult if either blockchain happens to be slow.

However, I think that it's a gross negligence not to watch out for any double-spending attacks. I think that the risk of double-spending attacks could be mitigated substantially if any client which sees a double-spending attack would warn the beneficiary. This would have to be done outside the normal broadcasting system because 1) nobody exept the beneficiary needs to receive the warning - so we don't need to flood anybody's network connection with this info. 2) Each client is supposed to bury the wrong transaction by not rebroadcasting it. REbroadcasting it anyway could encourage miners to include it in their blocks which is what we want to avoid.