Either the unvested shares won't vest immediately and things will go as you expected them to before acquisition, or they will all vest immediately, or they will vest immediately but be subject to a buyback. I've had all three in contracts.
If you're asking from the perspective of evaluating an options package being offered to you or if you're planning to offer options to employees, I'd say it's pretty usual to expect all options to vest in the event of an exit/acquisition.
It depends on what the options contract says, so read it and you'll see. Sometimes there is a provision for partial acceleration of vesting in the event of an acquisition.
Thanks all! So basically it should be in the contract how it should be handled in case of an acquisition.