Theoretically the current top tax bracket is 37% for anything over $523,601 -- I wonder if that's enough to be causing this already. There is, of course, the differential between this new "90%" (or whatever) bracket and the next highest bracket, which is currently 37%.

I'm curious how much people would do this. I suspect this would be very common for someone earning $120 million (give wife/child/best friend $20 million which would become $12.6 million after current existing taxes), but not common at all for someone earning $2 billion (they'd likely prefer to keep $290 million purely for themselves rather than keeping only $100 million for themselves and giving others $1.2 billion after taxes).