Interesting...

That is written for 2000 (this is the 2008 forum) - what it's saying is that because the inserted and deleted tables need to include all columns, not just the ones affected by the update, some operations may end up with increased logging information.

I've never heard of this before and I can't see any official reference that supports it, but it does make some sense. I suspect that the huge changes between 2000 and 2008 will have changed this one way or another, however I'll see if I can do some digging (if one of the heavy weights hasn't answered it definitively by then)