How Amazon ELB Health check Works?
I have the same problem. I just have it checking TCP:80 as an interim solution (which works fine).
I've come to the conclusion that the response, apart from being a HTTP 200 response, must contain certain headers. I've had a HTTP 200 returned from my tomcat server running on the instance not work, but a static html page served by httpd (also returning a 200 code) work fine. Looking at the headers, one of the few differences was that the tomcat one didn't contain the content-type. Not sure why that would make a difference, though.