Yep, concrete sinks ... usually. I remember visiting my Dad in Toronto in 1973 or 74 (he was on a temporary assignment there) and going to a park to watch college guys compete in a concrete canoe race. Huge team to get the canoe off a trailer and into the water, then paddle like mad to make very little headway. Wonder if that's been done since then.
Anyway, Chuck implied some very good advice. _The Family Handyman_ magazine has had a few articles about sinks and countertops made from concrete and other new / nontraditional materials. There are issues with them, but people can be happy with them. I's suggest you do a lot more research and reading before proceeding.
I just checked a homeowner site I'm also on
http://www.mlive.com/forums/homeimprovement/index.ssf and couldn't find any thread about it, but it's worth you asking if you're really interested.