I just got my new Reynolds Ouzo Pro full carbon fork. I noticed that the headcap for lack of my knowledge of the correct term seem relatively heavy. I am speaking of the cap that is inserted into the top of the fork. I am accustomed to using a star nut for my mountain bike forks, where as this thing seems to expand as the screw is tightened. Is there a lighter method/product? Is there a disadvantage to using a star nut with a carbon steerer tube?

You only need it to set the tension on the headset and once you are done it can be removed. Eastons come with a plastic plug that is very light for here is what you need for the ultimate in coolness. THIS IS IT! (http://fairwheelbikes.com/shop/index.php?main_page=product_info&cPath=15&products_id=56)

well, I'm pretty skeptical whether saving a few grams in this way is worth it, but I'd also make sure that swapping out a part like this doesn't void a warranty. My frame's integrated headset requires a specific headcap (from the manufacturer) and I'm pretty sure substituting another part would void a warranty. Since the fork is the weakest part of a frameset, I'd also like to make sure I'm not compromising any safety with such a part.

The Extralite UltraStar and Top cap.

http://www.sales.light-bikes.com/extralite/images/UltraStar1.jpg

http://www.sales.light-bikes.com/extralite/images/topcap_small.jpg

Total of 15g

My frame's integrated headset requires a specific headcap (from the manufacturer)
???? I have NEVER heard of this. What fork? Any quality expander plug should work.

cannondale's system integration extends to headset/fork and comes with a special headcap (with system integration logo and a warning not to replace or use non-SI component there), so I'm guessing if something happened to frame/fork with a non-SI part it might be enough to void warranty

To answer one of your original questions you CANNOT use a star nut in a carbon steerer tube, it will damage the steerer, and may lead to failure.