The link in /lib/modules/2.6.15-1.1878_FC5smp is incorrect. The link file build should point to /usr/src/kernels/2.6.15-1.1878_FC5-smp-i686, not /usr/src/kernels/2.6.15-1.1878_FC5-i686. I could not compile anything, until I renamed the directory in /usr/src/kernels/.
I did check bugzilla, and could not find a problem already opened up for it. I'm double checking to see if my search efforts failed :-)