Update @ 9:40pm, Fri. 07, 2011
This is a DIY on installing brake cooling ducts in an E85/E86 Aero front bumper. It will require permanently modifying the fender liner, as well as the faux grilles on the bumper.
I have not tested the actual temperatures at which these are effective; therefore I make no claims this DIY works any better than any other retrofit. It works as an air guide through the front cover to the front brakes directly. The steps below can be modified as you see fit for your own installation to better suit your own needs. An example of this is the amount of trimming done to the latter shown “port”. I have chosen to keep the length of the port quite long, thus giving a more finished look. Trimming the port length down will dramatically enhance airflow and, theoretically, performance (In this case, the port is angled in relation to the air flow entering it at speed. By shortening one side of the port, a constriction is removed, allowing a more direct channel for the air to flow through.).
This mod also requires the relocation of the ambient air temp sensor. I have chosen to go the E36 route and mount it in the brake duct itself. It is well protected, and still reads quite accurately.
You will also notice that the port tapers into the much larger inner duct. This means that during events where substantial amounts of water are entering these ducts (i.e. a car wash, following another car during a wet track event, etc…), water will run forward in the duct and into the bottom of the bumper. The lower corner shields of the bumper already have draining, and all electronics are up and out of the way. No damage should occur to your car… just know that this will happen. If you wish, simply add a drainage vent to the lower shield.
BE VERY CAREFUL AND TAKE YOUR TIME. It took me roughly 10 hours to do the test fitting for just one side, but I’m confident that following these steps will cut that time down to less than 4 hours for both sides.
Feel free to PM me if you need help or ideas for your own setup.
Again, this is my solution. I do not, and will not, claim to be the next Dark Schnitzer… If you have a better idea, please feel free to start your own thread. 
A full update will follow in the next day. This will include the how-to and pics.
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1) Purchase list…
a. Left Brake Duct - PN 51.74.7.138.413 (Note that the ETK diagram is incorrect. Callout #5 is accurate.)
b. Right Brake Duct - PN 51.74.7.138.414 (Note that the ETK diagram is incorrect. Callout #5 is accurate.)
c. Qty 2 - BMW liner bolts PN 07.14.7.129.160
d. Qty 2 - Port Kits, PN 268-352 from partsexpress.com (there will be left over parts)
e. Qty 2 - 24" zip ties.
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2) Consider also, but not required, list…
a. Universal Weather Seals
b. Weatherstrip Adhesive
c. Painter’s Tape