Not sure I can really help you much here, but let me try:
on your client, your code basically calls a method and passes it parameters. The WCF stack on the client side then converts that into a SOAP message (typically with an XML body, but could be binary, too) with headers and all and then sends that message across the wire to the server to be processed.
The server then attempts to deserialize that message into an object and attempts to call a message on a server implementation object. That method on the server object will most likely have the same parameters again, as the client - however, there's a possibility that the call will fail before that method even gets called.
So what I'm trying to say is: you can't rely on the fact that your server-side method with its parameters really gets called - there might have been a problem with e.g. authentication, the message format, a missing header or something else, that causes the server side to fail and throw an exception even before the server-side method ever gets called.
In the end, in the IErrorHandler, there's no way I would know of to get a hold of the message and/or the method and its parameters - all you can get are the error that occured on the server, and you can use that to turn it into a SOAP fault.
What you could do - both on the client and the server side - is create a new behavior that plugs into the WCF stack, and that records the methods being called and the parameters being passed into them - by implementing a class that implements the IParameterInspector interface of WCF. But that only will get called if the message on the client and the server will get properly deserialized and the server-side method really gets called.
Check out some of these links for more info on WCF extensibility:
Hope this helps a bit!
Marc