[4/14/07] I saw these in a local Asian ethnic grocery store in Minneapolis. If you know what this is please leave a comment.
[revised] It has taken exactly one year, but thanks to Danigirl we now know that this is a Trapa bicornis Osbeck horn nut!!! Say that 3 times fast.
"The seed has a crunchy texture with a bland flavour. It is rich in starch, the raw seed contains a deleterious principle that is destroyed by cooking - yea, right. The cooked seed can be dried and ground into a powder. It also has anticancer and antipyretic medicinal uses."
Many thanks Danigirl!