The kernel headers are stored in a different place for each version of the kernel:
Code:
cd /usr/src/kernels/`uname -r`-`uname -m`
should take you to the headers for the currently running kernel (assuming that's what you've installed). See what you've installed:
Code:
[david@malvern ~]$ ls /usr/src/kernels/
total 16K
drwxr-xr-x 18 root root 4.0K Jul 20 18:17 2.6.17-1.2157_FC5-i686
drwxr-xr-x 18 root root 4.0K Aug 10 20:41 2.6.17-1.2174_FC5-i686