Content-Disposition for streamed Excel reports

Greetings,

I am trying to write a WebForm that prompt for report criteria and on postback, return an Excel report generated using those criteria. I am using a 3rd party control that is working as expected to produce the Excel report, where I bump into trouble is that IE7 will correctly prompt for Open, Save, or Cancel the Excel document, it seems to be ignoring my Content-Disposition header.  I run the same page in Firefox and the streamed Excel file has the correct name. 
Below, is the code from a simple test version that tries to output a simple Excel workbook with "Hello World!" in cell B2.  Everything about it works except that in IE7, Excel is being prompted to open a file named Default.aspx and not a file named HelloWorld.xls.  The same file name issue exists for saving the file.



Is this a known IE7 issue or am I doing something wrong (see code below)?
Is there a way to have IE7 read the Content-Disp ...