Nothing pressing since the hardware isn't out yet, but there is a lot of stuff there that may impact things. One is an addition to the Win64 calling convention in that registers R30 and R31 (if available) are non-volatile. I don't know if ChezScheme still uses a custom setjmp and longjmp, but if so, that might need to change to account for that.
The other big thing is three argument instructions and potential optimizations around that.