The kid is just different. We've read all the sleep books and tried all the techniques. We dedicated, what? six months? to the silent-return-to-the-bed technique, and ... did I mention Gus is coming down every night and sleeping on the couch?
Every book I've ever read on parenting has said that consistency is key. Your kid must see you react the same way and do the same thing every time no matter what. If at first you don't succeed, try, try, again. And again. And again. And again.
If I'm not mistaken, this is also Einstein's definition of insanity. Doing the same thing over and over again and expecting a different result.
So I'm trying something new. It's called "not trying" (or, I suppose Type A people refer to it as "sleeping"). My body is still hard wired to wake up every 90 minutes, but in time that will change.