You need packet injection for Intel's 3945 card? Here's how its done.
First thing to say is the backtrack 2 kernel 2.6.20 is already prepared for these drivers.
1. Download the drivers
2. Extract the drivers
mkdir intel-drivers && cd intel-drivers
3. Compile & Install & Blacklist from startup
tar -xjf ipwraw-ng-2.0.0-10072007.tar.bz2
And now you should be ready to fire up your 3945 card in monitor mode. To start in monitor mode.
make && make install
echo "blacklist ipwraw" | sudo tee /etc/modprobe.d/ipwraw
Your Card name will be wifi0 not eth0 use wifi0 to inject. For eg
modprobe -r ipw3945
And to swap back to normal mode
aireplay-ng --test wifi0
I did just post this at the end of somebody elses post but I thought I'd post a guide for search engines ETC
modprobe -r ipwraw
This info was taken from the aircrack-ng site.