This is a slightly longer way of saying I'm not totally sure how what you're saying invalidates what valzam said: as far as the IRS is concerned, you did realize gains (you got something of value), just not on anything "liquid," hence AMT. Perhaps they (IRS) use different terms, but that's basically what's happening.
All of this to say, I think what you're saying is a much more precise way of saying what I was trying to get at. :)
>> If you hold actual stocks, there is no tax bill until you sell these to realize the gains
There are taxes to be paid as soon as you get the stock. If you are at google - have you noticed the number of RSUs which hit your schwab account are less than those which vested according to that chart in your schedule? That's taxes being withheld. The stock vesting is considered income at that moment and taxed as such.
(There is also a less important amount of tax due - by more ancient employees - which is about options.)