Map Virtual Directories To Page With Query String

See my last comment.

Here is how to handle post back situation.
I cut off some non important code, so the code might have errors.
It is easy to understand.
Together with the re-writer it will be working just fine.

public class TextWriterRewriteFormAction : System.Web.UI.HtmlTextWriter
    {
        public TextWriterRewriteFormAction(TextWriter writer) : base(writer) { }
        public TextWriterRewriteFormAction(TextWriter writer, string tabString) : base(writer, tabString) { }
        public override void WriteAttribute(string name, string value, bool fEncode)
        {
            if (name == "action")
            {
                HttpContext Context = HttpContext.Current;
                if (Context.Items["ActionAlreadyWritten"] == null)
                {
                    value = Context.Request.RawUrl;
                    Context.Items["ActionAlreadyWritten"] = true;
                }
            }
            base.WriteAttribute(name, value, fEncode);
        }
    }

And use it in a BasePage (your pages must inherit from it)

public class BasePage: System.Web.UI.Page
{
    private bool _rendered;

    protected override void Render(HtmlTextWriter writer)
    {

        if (!_rendered)
        {
            _rendered = true;
            using (StringWriter sw = new StringWriter())
            {
                BaseObjects.TextWriterRewriteFormAction newWriter = new BaseObjects.TextWriterRewriteFormAction(sw);
                base.Render(newWriter);
                newWriter.Flush();
                renderedHtml = sw.ToString();
                newWriter.Close();
            }
        }
        else
        {
            base.Render(writer);
        }
        writer.Write(renderedHtml);
    }
}

Good luck.

So here is what ended up working for me. We already have Umbraco installed, and as part of Umbraco comes http://www.urlrewriting.net/. That works fine with IIS6, but it didn't seem to handle postbacks well (the underlying URL would be revealed). So, I grabbed the Form.browser file described by this post and placed it in my App_Browsers folder. I also modified the web.config as that post states and I put the "FormRewriter.vb" file from that post in my App_Code directory. Since I am using UrlRewriting.net rather than UrlRewriter.net, I'm sure some of the web.config settings were not necessary (e.g., I setup the rewrite rules according to the methods required by UrlRewriting.net rather than UrlRewriter.net). Here is an example of a rule I used:

<add name="MyTest" virtualUrl="^~/UserSeesThis/?$" destinationUrl="~/ThisIsABackendPage.aspx"
    ignoreCase="true" rewrite="Application" redirectMode="Temporary" />

This was the content of the Form.browser file:

<browsers>
  <browser refID="Default">
    <controlAdapters>
      <adapter controlType="System.Web.UI.HtmlControls.HtmlForm" adapterType="FormRewriterControlAdapter" />
    </controlAdapters>
  </browser>
</browsers>

The FormRewriter.vb file contained this code:

Imports Microsoft.VisualBasic
Public Class FormRewriterControlAdapter
    Inherits System.Web.UI.Adapters.ControlAdapter
    Protected Overrides Sub Render(ByVal writer As System.Web.UI.HtmlTextWriter)
        MyBase.Render(New RewriteFormHtmlTextWriter(writer))
    End Sub
End Class
Public Class RewriteFormHtmlTextWriter
    Inherits HtmlTextWriter
    Sub New(ByVal writer As HtmlTextWriter)
        MyBase.New(writer)
        Me.InnerWriter = writer.InnerWriter
    End Sub
    Sub New(ByVal writer As System.IO.TextWriter)
        MyBase.New(writer)
        MyBase.InnerWriter = writer
    End Sub
    Public Overrides Sub WriteAttribute(ByVal name As String, ByVal value As String, ByVal fEncode As Boolean)
        ' If the attribute we are writing is the "action" attribute, and we are not on a sub-control, 
        ' then replace the value to write with the raw URL of the request - which ensures that we'll
        ' preserve the PathInfo value on postback scenarios
        If (name = "action") Then
            Dim Context As HttpContext
            Context = HttpContext.Current
            If Context.Items("ActionAlreadyWritten") Is Nothing Then
                ' Because we are using the UrlRewriting.net HttpModule, we will use the 
                ' Request.RawUrl property within ASP.NET to retrieve the origional URL
                ' before it was re-written.  You'll want to change the line of code below
                ' if you use a different URL rewriting implementation.
                value = Context.Request.RawUrl
                ' Indicate that we've already rewritten the <form>'s action attribute to prevent
                ' us from rewriting a sub-control under the <form> control
                Context.Items("ActionAlreadyWritten") = True
            End If
        End If
        MyBase.WriteAttribute(name, value, fEncode)
    End Sub
End Class

Here is the stuff that needed to be added to the web.config:

<configuration>
  <configSections>
    <section name="rewriter" 
             requirePermission="false" 
             type="Intelligencia.UrlRewriter.Configuration.RewriterConfigurationSectionHandler, Intelligencia.UrlRewriter" />
  </configSections>
  
  <system.web>
      
    <httpModules>
      <add name="UrlRewriter" type="Intelligencia.UrlRewriter.RewriterHttpModule, Intelligencia.UrlRewriter" />
    </httpModules>
    
  </system.web>
  <system.webServer>
    <modules runAllManagedModulesForAllRequests="true">
      <add name="UrlRewriter" type="Intelligencia.UrlRewriter.RewriterHttpModule" />
    </modules>
    <validation validateIntegratedModeConfiguration="false" />
  </system.webServer>
  <!-- I actually left this out, as I wasn't using this rewriter. -->
  <rewriter>
    <rewrite url="~/products/(.+)" to="~/products.aspx?category=$1" />
  </rewriter>
  
</configuration>

Our server is running .Net 4, so that might be important too. This solution worked for extensionless URL's and for postbacks, so it satisfied all of my requirements.