[Edited to correct my stupidity about the Unicode thing]

<sigh>  After a few frustrating hours trying to get the full-text indexing search in Sql Server 2005 to use the darn thesaurus file I was writing, I happened upon this newsgroup post in which the guy was having the exact same problem as me.  In essence, Sql Server expects a unicode file with byte order marks written to it, which TextPad doesn't do by default.  Changing TextPad to save as unicode with byte order marks solves the problem.

I'm hoping the lots of superfluous and gratuitous keywords added to this post (like "Sql Server 2005", and "full-text search" and "thesaurus" "not working") will mean that someone else out there is saved the trouble I had.