I found a patch for kernel 2.6.25.8 that supports SMP863x devices.
Here's a link:
http://download.sagetv.com/pubcode/HD200kernel.patch
Edit: The source seems to be pretty complete.
very strange. it seems that this patch is not an official patch by Sigma, as there is not 2.6.25 kernel source on the Sigma FTP.
I noticed that, too. I wonder if that SageTV HD Theater is worth the money?
http://www.freytechnologies.com/hd_theater.html