SqlDataSource.Select Is Working, But Assigning Null

The problem I'm having is described below all this code.

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My content page has a SqlDataSource:
<asp:SqlDataSource ID="sqlGetUserInfo" runat="server"
ConnectionString="<%$ ConnectionStrings:RemoteNotes_DataConnectionString %>"
SelectCommand="SELECT [UserFirstName], [UserLastName] FROM [Users] WHERE ([UserGUID] = @UserGUID)"
onselecting="sqlGetUserInfo_Selecting">
<SelectParameters>
<asp:Parameter Name="UserGUID" Type="String" />
</SelectParameters>
</asp:SqlDataSource> 

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Inside my OnSelecting event, I have: protected void sqlGetUserInfo_Selecting(object sender, SqlDataSourceSelectingEventArgs e)
{
sqlGetUserInfo.SelectParameters["UserGUID"].DefaultValue = Membership.GetUser().ProviderUserKey.ToString();

} 
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And, inside my Page_Load, the relevant code is:
//String strUserFirstName = ((DataView)sqlGetUserInfo.Select(DataSourceSelectArguments.Empty)).Table.Rows[0]["UserFirstName"].ToString();
DataView dvUserDetails = (DataView)sqlGetUserInfo.Select(DataSourceSelectArguments.Empty);
if ((dvUserDetails != null) && (dvUserDetails.Count > 0))
{
DataRow drUserInfo = dvUserDetails.Table.Rows[0];

lblHelloMessage.Text = "Hello, " + drUserInfo["UserFirstName"].ToString() + ((drUserInfo["UserLastName"].ToString() == "") ? "" : " " + drUserInfo["UserLastName"].ToString());

}  
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Now, here's the problem. My "if" statement is never executing because "dvUserDetails" is null. However, when I break the execution and put a
watch on the actual Select() statement, it shows a DataView with the correct return rows! You can see the commented line where I tried to
bypass the DataView thing (just as a test), but I get an object reference is null error.

The weird thing is that it was working fine one minute, then started getting "funky" (working, then not, then working, then not), and now it just doesn't work at all. All this
without me changing one bit of my code because I was checking out some UI flow and stopping and restarting the application. I've tracked down
the temporary files directory the localhost web server runs from, deleted all those files, and cleaned my solution. I've even tried
rebooting, and nothing seems to make it work again.

My relevant specs are VS2008 9.0.21022.8 RTM on Vista Enterprise x64.