Apparently, Mclaren say that this part broke on Hamiltons car just before he hit Alonso on sunday, which was the cause of his sudden increase in speed which made him hit the renault.
Apparently the loss in drag was enough to increase the acceleration.
http://www.formula1.com/news/headlines/2008/4/7615.html
If you look at the lower picture on this page ^^^ you can see that it has broen off of lewis's car.
So it must provide 'some' downforce.
