I guess you and maybe also John are talking about the IIS logs. I have no solution for that, it is outside the scope of mojoPortal. When I said I have implemented database logging I was talking of the mojoPortal log which only logs errors and other information logged from code. We use log4net for such logging and historically the logging has to been to a file at /Data/currentlog.config and is viewable from Administration > System Log
All I have done is implement a custom logging appender for log4net that logs to the database so that we can have one central error log for all nodes.
From a quick research it doesn't sound like there is a way to log multiple web site nodes to the same IIS log. The suggestions seem to be that one could use logparser to process the separate log files into a database. In fact they say it is better for each node to log separately so that if there is a problem on a specific node it can be identified. Even if there were a way to do it, it would have to be something outside of mojoPortal.
Just on a side note, one interesting tool I've seen lately is this free Log Parser Lizard that provides a nice GUI on top of the Microsoft log parser and allows querying against the IIS log files. Scott Hanselman has a nice blog post about it.