The patch in the included URL (comment #15, the second patch posted) fixes a showstopper problem when using DomainKeys validation with spamassassin. All that is needed is adding the patch to the FILESDIR and running epatch against it in the src_compile() function.
Created attachment 81259 [details, diff] backwards-compatible.patch Probably a good idea to attach patches to the bug. It's mainly done for archiving purposes, and having one place to fix the bug and pull patches from.
3.1.1, which should be in the tree this afternoon, has this patch already built in. Marking fixed.