Ah, I figured out the VIHH thing. It's Vdd + some delta-V used in order to use their "wiperlock" technology. You can only activate it if CS exceeds Vdd.

Regardless, I still can't adjust the resistance of the chip. I'm using the following code with a 16F628:
---------------------------------
cmcon=7
define osc 8

res var byte

CS var porta.2 'CS
CLK var porta.3 'CLK
SI var porta.4 'SI
LED var porta.1 'LED

high CS
high LED
pause 500
low LED
pause 500
high LED

repeatit:
res=24
low CS
shiftout SI,CLK,1,[%00000000\8,res\8]
high CS
pause 100
low CS
shiftout SI,CLK,1,[%00010000\8,res\8]
high CS
pause 200
res=res+128
goto repeatit

end
---------------------------------

I have a 10k pullup resistor on SI, yet no change in resistance.

Any advice or help is appreciated!

Dave