I found these sites. I do not recommend the use of the chemicals recommended here. The browning seems to resemble Alternaria blight as noted here.
http://ipm.ncsu.edu/current_ipm/96Pe.../ornament.html

I found this interesting.
http://www.wezel.com/garden/articles/poinset.htm

The white looks suspiciously like powdery mildew. The brown spots can also be a result of that as well.
http://www.caes.state.ct.us/PlantPes...hP/pphpoin.htm

I would suggest the use of a fungicide that you can make your self along with disposing of any dropped leaves as soon as you see them. Any of the first three here would work. If you think you have one of the fungal diseases going on as well, use the third recipe with the citrus oil.
http://www.dirtdoctor.com/view_question.php?id=204

Hope this helps,
Newt