This is comforting - I stand confirmed, as well as Yasser (especially his great article in MSDN Magazine) and many, many others, too. ASP.NET's [WebMethod] feature is very cool, but I must say that it can lead to sham ... you really tend to see the Web Services world just as another OO or RPC thingie. No! It is all about messages. Period!
Read Tim Ewald's excellent MSDN article on XML Web Services as a message based communications means - OK, the main tenet is about showing how to access the raw SOAP message in ASP.NET Web Services (which is really interesting stuff, I also have a working implementation for this). But the really important message is in the few sentences in the introduction - be sure not to miss them!