[theIV]

If they are ISOs, tax won't be due _upon_ exercise but will show up on that years tax return. Usually the AMT will hit you (if the exercise was worth it), and you'll owe the following year.

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.

[iav]

when the gain shows up is irrelevant - IRS requires taxes to be paid on income in the quarter it is realized. Everyone needs to pay the estimated tax and then file the return for the final adjustment.

[theIV]

I'm not super familiar with this aspect, so this could be totally off-base: because this is sort of "out-of-band" income, it might factor into what you should be paying, but you won't be penalized for it; you didn't know exactly what the estimated income was (i don't believe you have to get a 3921 immediately upon exercise) so you can't necessarily estimate the taxes owed. This could be totally wrong? Also, I think there's a lookback period of a year that you can base your estimated taxes on.

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. :)