Thanks to Patrick for finding this really interesting post by Joel Kallman.
Finally, mod_plsql has been ported to Apache2. Up until now it was only available and supported for the Oracle Http Server based on Apache (1.3.x).
Using this configuration on a *nix (Linux / Unix) based machine would start a new database session even if the request would only serve a static image. Thus the connection pool of mod_plsql wasn't used at all. This is the first time where I have seen an advantage of the Windows architecture over Linux, bummer.
Thus on Linux you had to be careful planning for your environment if you expected a heavy load, here are some of the typical things to do optimize the performance.
Using Apache 2.0 and the new mod_plsql this should be improved now.