Several of the blossoms have opened all of the way. Two days ago it was still missing the center - http://www.pbase.com/ed_k/image/53022503 --- Quite a striking flower.
I stuck a circular polarizer on my Nikkor 105mm micro (macro to every other manufacturer, micro to Nikon). Portions of the catcus's petals tend to turn white in the photo from reflected light. The polarizer, and White Balance set to shade (3 steps to the warm side) nailed the colors. Of course, shooting RAW the WB wasn't critical, but it's nice to get it right in the camera when you can.