Using Both Forms and Windows Authentication

I have an ASP.NET application that targets the 3.5 Framework SP1. The application is configured for Forms Authentication but I would also like to be able to grab the ID (HttpContext.Current.User.Identity.Name) of the logged in user. In ASP.NET 2.0 I set the Web config file to use Forms Authentication and configured IIS for Windows Authentication. This worked fine until I upgraded my application for Visual Studio 2008 and the 3.5 Framework SP1. Is this method not allowed in newer versions? If not, then how can I resolve this issue? Thanks.
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