Tracing in ASP.NET : solved !
During these "vacation" days I studied the Tracing capabilities for a web application (both in ASP.NET 1.1 and 1.2). The solution is pretty simple, ASP.NET doesn't works on System.Diagnostics.Trace class but on System.Web.TraceContext. Then, if you need to trace something with ASP.NET (Web Services too) you have to use TraceContext's methods (Write and Warn).
In order to write ASP.NET trace messages to some trace listener you have to set writeToDiagnosticsTrace to true (<trace enabled="true" writeToDiagnosticsTrace="true"/>). In this way, all messages (using both Trace or TraceContext) will write to the listeners defined.