>I created an Excel file at runtime using the StreamWriter, and saved the file as an XLS file. When I try to open the file on the PDA I get a message stating that it is an invalid file format. When I copy the file to my PC and dbl-click on the file it opens up in Excel just fine.
>Is there a limitation on Windows Mobile Excel that causes it to not open an Excel file crated using XML tags?

To open it on a PDA I think you'll have to save it with a .pxl extension.