I've tried with fakeroot debian/rules binary-arch-headers but it returns an, IMHO, absurd error:
mkdir /home/simon/sources/kernel/ubuntu-natty/debian/linux-libc-dev/usr/include/
mkdir: cannot create directory `/home/simon/sources/kernel/ubuntu-natty/debian/linux-libc-dev/usr/include/': File exists
make: *** [install-arch-headers] Error 1
The error is correct but this folder is created in the same process!!