Logging Timeout'd Request in Apache 2.X
I found this in the Apache mailing list https://issues.apache.org/bugzilla/show_bug.cgi?id=39785. Which talks about this. This looks like it was resolved in apache release 2.2.2. This is the relevent check in http://svn.apache.org/viewvc?view=revision&revision=919323. It has a comment stating "Only log a 408 if it is no keepalive timeout."
So check to see if you have a value for KeepAlive set and check to make sure you're at version 2.2.2 or higher.