having worked in the past as both a screener and a phlebotomist for Great Lakes Blood Bank, affiliated with ASWBPL, I can definitively say that well over 90% of the blood used for "combat troops" is obtained from US military blood donation centers and blood banks. The vast majority of blood given to stateside service members and their families is received from numerous source to include ARC, Blood Services USA, and a bunch of others.
Thus ones blood that is donated to the ARC (or other such agency) is TECHNICALLY given, at least a very small percentage, to service members. The down side to this is that most of it is given to either those service members who are having elective surgeries or the ones going through certain types of blood treatments.
Very, very, very little of the ARC blood actually makes it into the combat zones or to the front line troops.
That being said one can donate to the troops directly by getting a hold of your local military or VA hospital. Even if they do not have a "blood cadre" they can point you to the place that they purchase their blood from for surgeries. (yes purchase, with tax money...!)
I know that when I was on the Great Lakes team we would average 2 -3 drives a week. 1 at the "boot camp" side getting all the freshly scared boots to "volunteer" to give blood to get out of a days worth of getting their butts physically PT'ed to death. The second would usually be on the "advanced training" side where the recruits would go after boot camp. This setup worked out nice because it was roughly 2 months of boot and then they were "qualified" to give again. If we did a third one in a week we would try to do it somewhere that spouses and even members of the "non-military community" could come out and donate. This may have been places like the base housing gym (not really on base so civilians could get there too) or the gas station outside base or any number of other locations.
Anyways, sorry for the long rant and rave. Bottom line is if you are still concerned for the troops in Afghanistan and want to try and make sure your blood goes to them your best bet is to contact your local military hospital, whether that be Army, Navy or Air Force. Another option would be the VA, although the majority of this goes to veterans (the ones who have already been kicked out/retired cause their arm or leg got blown off). ARC and others will not be able to even give you a glimmer of hope of helping a wounded 18 yr old kid who got his arm blown off in the dusty town of kalamamala...