So this is totally killing me.
I've found out how to make a tablix header row repeat on each page of a PDF export file in SSRS 2008 (which I won't even get started on because all I can say is that 2005 is way better), but I cannot figure out how to make a simple header row of a tablix freeze in an Excel export file. The tablix is right at the top of the body and spans the entire width of the design area. How in the world do you get it to freeze in the Excel export file?