After methodically following the tutorial all the way to step 30 I happened to turn the case over and heard the distinct sound of a trinket of metal hitting the hard wooden floor with the sound of a tiny high pitched drum roll as the culprit scurried across the floor. Finding the component was easy but trying to figure out where it came from proved to be more difficult. About an hour of analysing the photos in the tutorial proved futile and I eventually decided I had to assemble what I had, and see what screws I would be left with in a bid to determine where it used to live.
At the end of the re-assembly I was none the wiser about how this odd shaped component with an internal thread would factor into the scope of the iPhone 4S design and I decided to see if it would power up anyway. Alas, after reconstructing the phone it failed to even offer a hint of power or display. There was definitely no reference to it in the guides either :(
Turns out, perhaps from one of those heart wrenching phone drops from the table to the floor, that this piece must have dislodged from the chassis of the case assuming it was actually attached at all. I located it's slightly hidden location in the top centre of case normally hidden underneath the logic board and was able to verify the positioning of the part in the photo of Step 25. It looks like it has a couple of little circuit contact points which align with some metal on the logic board. This may account for the lack of response in the unit once I had fully reassemble the unit. The internal thread section accommodated the 2.7mm screw used to secure the cable cover of the logic board. After this piece was back in alignment the phone fired up as expected - with a huge sigh of relief might I add :)