I just confirmed with the NUC team that 1066, 1333, and 1600 memory should all work but will run at 1066 regardless. The confusion (for me at least) was that since the memory has to be low power (i.e. 1.35v) you're not likely to find 1066 memory that will work.
I recommend checking your memory's model number against the compatibility list as not all memory is fully compatible or tested. I've never tried PNY memory in mine.