OK, after trying a bunch of different ways of exporting video out of Final Cut Pro and getting it into YouTube, I have a new workflow. It isn't he fastest or the prettiest, but it gets the best results.
Step 1: Export out of FCP (however you want, just make sure it will work with Step 2)
Step 2: Import into iMovie '08.
Step 3: "Share" to "YouTube".
I didn't keep the other versions of the video on YouTube for your comparison, but this to me made the best bang for your buck. It is quite softer than the other ways of doing it, but you trade the softness for quality and pixelation.
OK, my words are rambling because it is late. You try this workflow out, and tell me if it works for you.
Oh, and:
Matthew