At SBL PNW I was talking with Jack Levison and his student Bill Horst. When Gen. 6.3 was brought up both of them were a bit surprised by my reading. This verse says:

וַיֹּאמֶר יְהוָה לֹא־יָדֹון רוּחִי בָאָדָם לְעֹלָם בְּשַׁגַּם הוּא בָשָׂר וְהָיוּ יָמָיו מֵאָה וְעֶשְׂרִים שָׁנָה׃

And YHWH said, “My Spirit will not dwell in humanity forever, because he is flesh. And his days will be one hundred and twenty years.” 

I understood it to mean that God has withdrawn his Spirit from the midst of humanity and this greatly shrinks human lifespan. They understood it to mean that (on average) the spirit of God (as life-force, a la Gen. 2.7) only dwells within humans for one hundred and twenty years before departing causing death.

How would you read it?