webService.SoapVersion =
System.Web.Services.Protocols.SoapProtocolVersion.Soap12;
[Edit 9 Dec 2010]
Actually, this was a red herring. It worked by adding a "service reference", rather than a old-fashioned "web reference".
ASP.NET MVC, SQL, Visual Studio, C#, Azure
webService.SoapVersion =
System.Web.Services.Protocols.SoapProtocolVersion.Soap12;