ok, I figured it out (sort of). There's a page with version info here (of course they don't tell you how to determine what version you already have). The latest version is currently 3.01 and it supports adding UNC paths to search.
They claim there are only two versions of Desktop Search - 2.66 and 3.01 but I had the version that comes with Office 2007 and still couldn't add UNC paths so obviously there are sub-versions that aren't listed.
This was going to be a big concern for me since I do a lot of searching for specific files (music tag info, archives, etc) and I'll be moving a lot of that data over to WHS so I wouldn't be able to search it locally until I added the Desktop Search update with UNC paths.