We started dating October of 2007, got engaged December 2009, and will be married on our 4 year anniversary October 2011.
We just knew too. We were "seeing each other" for about 3 weeks. When I say I was seeing him, I maybe saw him 3 times in those 3 weeks. haha. Then we ended up in this terrible incident where my FI was jumped by about 10 guys. He was hurt badly. Only our 2nd date technically. I was so worried about him, I stayed over for the first time and took care of him. I never left after that. I slowly started moving my stuff over, until we had to move to accommodate all of my stuff. But we loved every moment. I have had other very serious relationships, and like you said, none were like this. He had never lived with someone else, until me. We just meshed so well that we were like two peas in a pod after that one night. Love him to death!