Definitely not a good sign for Chuck...though its also not the end of the world for Chuck either. I am still cautiously optimistic. At any rate, of all the shows on NBC this year, even if Chuck gets cancelled, I'd say it is the best candidate for the now coveted DirecTV bailout option.
Also, I think you're right. If Leno bombs, NBC can always order more episodes. Just because the initial order is 16 doesn't mean they can't say "Hey, we'll take six more." I personally think Leno is going to bomb hard and CBS' ratings will go through the roof as a result.