I have a Silverstone case that came with installed fans, and these fans have posts on each corner. The Thermalright HS is designed for fans that do not have posts (also known as 'Flange-type fans).
You can remove the posts with a Dremel (or saw) without too much problem. You only need to cut away enough for the hole to be exposed.
I have no idea how to explain this in words, so here's a pic:http://img237.imageshack.us/img237/2848/fanpostsij5.th.jpg
This was my 1st attempt at exposing the hole. In later attempts, I cheated and just used a saw to cut a triangle out :P