Acceleration usually comes as single-trigger or double-trigger.
Single-trigger is what you are talking about: when all of your options vest immediately upon acquisition. It could be argued that this is unfair to those that have worked their full time to earn their full options grant. Usually, in this arrangement, a certain percentage of your shares are subject to the trigger (so, 25% vest immediately, for example). I think it would be unusual for 100% of your options to vest immediately upon acquisition: what if you were acquired a month after you joined?
The second type is double-trigger. Let's say that you have 75% of your options unvested in an acquisition, and you have 3 years remaining to vest at the new company. If they fire you, you lose the rest of your options. Double-trigger acceleration is where your options vest immediately if you are not fired "for cause". That means that, as long as you are not being grossly negligent at your job, the new company can't screw you out of your unvested options by laying you off.
The one thing to remember here, is that a lot of this is subject to your leverage over the company and the leverage the company has with its new acquirer. All of these terms are subject to renegotiation in an acquisition, and anything could change at any time -- look at what happened with Zynga.
Consider it a gentleman's agreement, for the most part. Good people will honor it, shitty people might try to screw you. As with anything, you should only do business with those you trust.
And finally, the absolute best way to make sure you get the full value of your options (as an employee or a founder) is to always make sure you are indispensable to the company. A company who needs you can't screw you.
(final note: IANAL, this is my understanding of how things work)
When you combine this with the famous Charles de Gaulle quote ("The graveyards are full of indispensable men"), you properly understand the predicament.
So in the case where you don't have accelerated vesting, what happens to unvested options in an acquisition? Do they get converted to options in the equivalent dollar amount of stock in the acquiring company on the same vesting schedule? (Obviously subject to negotiation, YMMV etc etc...)
With an acquisition I went through, none of the unvested options (after acceleration) turned into anything (meaning those remaining shares were never issued/created). However the acquiring company put forth their own stock incentive plan in hopes of retaining employees.
Single-trigger is what you are talking about: when all of your options vest immediately upon acquisition. It could be argued that this is unfair to those that have worked their full time to earn their full options grant. Usually, in this arrangement, a certain percentage of your shares are subject to the trigger (so, 25% vest immediately, for example). I think it would be unusual for 100% of your options to vest immediately upon acquisition: what if you were acquired a month after you joined?
The second type is double-trigger. Let's say that you have 75% of your options unvested in an acquisition, and you have 3 years remaining to vest at the new company. If they fire you, you lose the rest of your options. Double-trigger acceleration is where your options vest immediately if you are not fired "for cause". That means that, as long as you are not being grossly negligent at your job, the new company can't screw you out of your unvested options by laying you off.
The one thing to remember here, is that a lot of this is subject to your leverage over the company and the leverage the company has with its new acquirer. All of these terms are subject to renegotiation in an acquisition, and anything could change at any time -- look at what happened with Zynga.
Consider it a gentleman's agreement, for the most part. Good people will honor it, shitty people might try to screw you. As with anything, you should only do business with those you trust.
And finally, the absolute best way to make sure you get the full value of your options (as an employee or a founder) is to always make sure you are indispensable to the company. A company who needs you can't screw you.
(final note: IANAL, this is my understanding of how things work)