It was added in a patch to 2.6.23

Notes from the coder:

The bonus feature in my patch series adds the magic file /sys/kernel/notes. Reading this gives you the binary contents of the ELF notes section built into the kernel. Here you can find the build ID of the running kernel. This gives a solution to a problem that has arisen for systemtap users, where nothing prevents them from using the kernel-debuginfo.i586 data to drive Systemtap’s probe details, but are actually running the kernel from the kernel.i686 rpm. This is a failure on many levels, but some simple sanity-checking at the bottom always helps. Now it is easy to verify you have the right debuginfo file for the kernel you are running.

This is incorporated upstream now.