Lennart Poettering has written about systemd's Forward Secure Sealing functionality on his Google+ page. Hackers can still delete the log history to cover-up their intrusion, but they cannot alter them as systemd's FSS uses a cryptographic seal of the system logs at regular intervals. "It works by generating a key pair of "sealing key" and "verification key". The former stays on the machine whose logs are to be protected and is automatically changed in regular intervals (and the previous one securely deleted), the latter should be written down on a piece of paper or stored on your phone or some other secure location (that means: not on the machine whose logs are to be protected). With the verification key at hand you can verify the journals on the machine and be sure that -- if the verification is successful -- log history until the point where the machine was cracked has not been altered a posteriori."
The Forward Secure Sealing is interesting, but many will still need to rely upon an external secured log server since systemd cannot guard against the system log files on the local machine from simply being removed. The FSS feature is based upon the Forward Secure Pseudo Random Generator, which is a cryptography project being done by Lennart's brother at university.
The FSS code is already living in systemd's Git code-base for its next release.