I have just written about how I plan to implement digital signatures in LogStore, the secure store used by LogTools. The log store digital signature proposal details how and when signatures are written and provides reasoning why it will probably happen this way. There are two goals for the proposal: one is to document how things will work and the other, probably more important, one is to draw some feedback. It is easy to get security tools wrong, and even those with highest experience in that area (which I have not!) can fail. So it would be very beneficial to have some other folks read the proposal and comment on weaknesses they find - or simply things they would do differently or add to the overall idea. With that said, please read the (small) paper and provide feedback ;-).
Please keep on your mind that his is not only related to syslog but can be used with any text-based log (including binary logs that are converted to text, e.g. by base64 encoding them). So it can affect you even if you are not interested in syslog itself. My (mostly uneducated) assumption is that this could be a toolset of great use for computer forensics.