One thing no one has mentioned yet, so I may as well......
I've been advised several times that when sitting in seiza for a long period of time, not to get up quickly, take as long as is required to get feeling back in the feet and legs and let no one ruch you.
At one dojo I went to I was told that they'd had at least one case of a broken toe and a Japanese friend told me of a couple of cases of broken ankles when people got up too quickly.
Also, for what its worth, sitting in seiza can become "Comfortable" after a while. I practiced at home every day, slowly building up the length of time in seiza while watching TV or reading. After about nine months of this sitting in seiza is comfortable. The problem still remains of my feet going to sleep and after about 30 - 40 minutes I lose feeling in them. Does anybody have any suggestions for preventing this (apart from getting up).