I did this in the last census. I got a few nasty phone calls demanding a real answer of what my race is, then they sent someone to my door. He asked for me by name, then checked me off as the race that he saw me as. My mom is a genealogist and has uncovered people of nearly every race in our past, so this makes these questions really seem stupid.
Since that time, I married a girl who is half white and half filipino. We have 2 daughters that are both the same amount filipino. One is darker and it would be hard to place an ethnic classification on. The other is fair, blonde with blue eyes and I guess they'd make an assumption on her too.
I don't understand what these questions are for, but I had a poly sci prof that told us that the government uses the numbers for allocating funds and to make sure there is a "fair" distribution of wealth.