journald's Forward Secure Sealing is an easy to use integrity mechanism that was previously unavailable in text-based syslog implementations... of course it cannot prevent your adversary from deleting all the logs, but at least that can be reliably detected.

http://lwn.net/Articles/512895/

And rsyslog can cryptographically sign logs too nowadays:

http://blog.gerhards.net/2013/05/rsyslogs-first-signature-pr...