Trips to the bathroom tend to use a lot of water, so anything that saves precious H2O is probably a good idea. This sink and toilet combination achieves that by using the waste water from the sink to flush the bowl.
Korean designer Jang Woo-Seok's Eco Toilet concept has a special greywater tank that collects the waste water from washbasin, then uses it to flush the toilet. Of course people are supposed to wash their hands after using the toilet, so any greywater would be loft over by the last user. Just in case they're one of those filthy people who don't wash, the toilet can also switch to freshwater for the flush if needed.