The original poster is correct: When you create a file through some external means and save it as a .xls file, by default "Show Gridlines" is turned off. This means that your data appears on a plain white background. You can manually turn on gridlines (Tools >> Options >> View >> Gridlines), but most users won't know how to do that (or won't think of it).
Anybody know if there is a way to do this programmaticly?
Yes, of course you can slap literal borders on your grid, but that just adds to the original problem: You still have gridlines turned off, but now you also have a bunch of formatting all over your cells. I'm trying to avoid that.
In my case, I am exporting the sheet from HTML via a dynamic script. The data is in a no-frills table, and I set the MIME type to application/vnd.ms-excel. Works great, except for this one problem.