By default, RH6 installs a kernel-headers RPM package that contains kernel
and module symbol information for non-SMP kernels.  If a user installs an
SMP kernel that ships with RH6, modules can not compile and load against
the SMP kernel because the symbol information compiled within the module
does not match the exported kernel symbol data.

Rebuilding the SMP kernel from scratch (i.e. starting with 'make mrproper')
creates the correct data in /usr/src/linux/include/linux/modules, which
then enables modules to compile and load against the SMP kernel.  It would
probably be better to include either another kernel-headers RPM package for
the SMP kernel, or add SMP symbol data to the main kernel-headers package
(like what seems to have happened with RH6.1).