It seems to me, there is no kernel 3.1.9+ source deb package in Raspbian repository. According to Alex Bradbury (asb) they built the Raspbian kernel based on https://github.com/raspberrypi/linux. Based on this kernel compilation documentation I did the following (as root):
cd /usr/src git clone --depth 1 https://github.com/raspberrypi/linux.git ln -s linux linux-3.1.9+ cd linux zcat /proc/config.gz > .config ln -s /usr/src/linux /lib/modules/3.1.9+/build
After that I was able to recompile the external module.