I have working Fedora 9 x86_64 and i386 machines. Today the kernels on both machines were upgraded to 2.6.25.10-86. After this the x86_64 machine worked fine, but the i386 machine had all the header files in /usr/include/asm jumbled up. That is, the file names and the file contents did not match. I found a copy of kernel-headers-2.6.25.9-76.fc9.i386.rpm, and put that back on the i386 machine. I can now successfully compile my code once again.
It looks like this was an unfortunate coincidence. It seems there was minor corruption of the file system. After a full fsck, and a cleanup of the couple of issues it found, the kernel-headers RPM installed OK. Of course, that doesn't explain how the file system became corrupted without any hardware errors being reported, but it seems to been the current bug report is spurious.