Michel why don't you use a soap extension, that's how I do it, I created a dump soap extension that plugs into the soap pipeline and log's all the calls input, output of the soap messages into a file.
Soap extensions are very powerfull and fun to work with once you get the hang of it.

>In my logging mechanisms, to log the parameters received into a Web Service method, I am calling a method at the end of each method for the logging purpose where I pass the parameters received. I would like to know if there would be a way to use the Request.Headers() to detect the parameters received in order to avoid doing all that manually at the end of each method.