This blog post by Shannon Hicks shows how the problem was traced to a glut of hundreds of log files in /private/var/log/asl. [crarko adds: I just looked at my Mac, and there are only about 25 files in mine. That seems to be OK.]
If you don't need to keep these logs for some other reason, they can be deleted using the Terminal command:
sudo rm -f /private/var/log/asl/*
[crarko adds: I haven't tested this one. If you don't have the issues described, it's best just to leave the log files alone. See the comments below for more information about the log rotation process.]