This patch will allow a non-root user to build a rpm
for a specific kernel version if that kernel is
installed on the system. (You don't need to be running
that kernel version either)
make rpm KERNELTARGET=2.6.8-1.521
This is only an example and has not been tested to see
whether it works with 2.4 kernel builds.
This procedure works in my test system for 2.6 kernels
on Fedora Core 2.