Yep, building under scratchbox is a little bit tricky.
After executing :

$ GIT_SSL_NO_VERIFY=1 git clone https://vcs.maemo.org/git/kernel-power
$ cd kernel-power
$ kernel-power-2.6.28/debian/rules get-orig-source

you have to cd to kernel-power-2.6.28 and

dpkg-buildpackage -rfakeroot -S

to build a source package.

then
cd..
rm -rf kernel-power-2.6.28
dpkg-source -x kernel-power_2.6.28-10power49.dsc