I've been plagued by this one problem for a long time now. As it came across today, I pinpointed it. If you're using a new kernel (errata 2.2.16-3, ext2 2.2.17-5) on older system (RH60), compiling SMP modules (e.g. external network drivers from ftp.scyld.com) will fail because SMP related issues are not defined and modules are compiled for UP. Manually defining -D__SMP__ or friends won't help either because other stuff is left out too. This is caused by the fact that those errata kernels don't Require: initscripts new enough which create /boot/kernel.h at boot. Updated initscripts would be probably too much to ask, but _please_ at least Require: decent initscript versions :-)
*** This bug has been marked as a duplicate of 20025 ***