Hi.
For cross compiling: There is a *very* good cross compiler
HOWTO on http://bytesex.org. It's for i386->sparc, but
based on that it is easy to create a cross compiler for
the big iron. (I've already created one, if you want it
I can give it to You. I've succesfully cross compiled
31bit kernels with IBM's 2.4.17-whatever-stream patches.)
For the debian part: I've never used kernels from
debian sources (nothing personal, I just like kernel.org :-)),
but is should be the same to cross compile a debian kernel with
make-kpkg (as far as i know make-kpkg is some kind of architecture
independent script).
Hope it'll help.
KK