Adventurous users, contributors and developers can enable the Integrity Measurement Architecture subsystem in the Linux kernel with appraisal (since Linux kernel 3.7). In an attempt to support IMA (and EVM and other technologies) properly, the System Integrity subproject within Gentoo Hardened was launched a few months ago. And now that Linux kernel 3.7 is out (and stable) you can start enjoying this additional security feature.
With IMA (and IMA appraisal), you are able to protect your system from offline tampering: modifications made to your files while the system is offline will be detected as their hash values do not match the hash values stored in extended attributes (whereas the extended attributes are then protected through digitally signed values using the EVM technology).
I’m working on integrating IMA (and later EVM) properly, which of course includes the necessary documentation: concepts and a ima guide for starters, with more to follow. Be aware though that the integration is still in its infancy, but any questions and feedback is greatly appreciated, and bugreports (like bug 448872) are definitely welcome.