I knew in my heart that this was true. I had heard it several times, but darned if all of my personal tests just seemed to support the idea that the result set was always returned in order by the Clustered Index fields. But now, I have the proof I have been seeking!
In a comment to my How to Return Record #n post, Hugo Kornelis posted a link to his own post with the evidence (and later an explanation). I thought this was such a big deal that it deserved to be highlighted in a primary posting for the benefit of all those who missed his original post, only read the RSS feed and never follow the comments.