[HostFat]

You can instant accept any bitcoin value that is lower to the investment to do the 51% attack or similar. Example: no one is going to make a 51% attack to steal only 10 bitcoin from a vending machine.

Simply put a limit, or just say to the user that any amount of bitcoin over X will need Y time to get verified. Anything lower that X amount will be instantly processed.

[fryguy]

Doesn't it require ~10 minutes to solidify into the record to prevent double-spending without having the 51% attack?

[elliottcarlson]

Correct - there are various attack vectors besides the 51% attack. Ideally, a transaction shouldn't be closed until there have been 2 confirmations.