The reason you 'never saw a .us showing up first in SERP pages' has nothing to do with how search engines treat them.
Pick a keyword. Then, do some competitive keyword analysis. See how many .us names are developed in that space. If there are few to none (or, those that are splogs), then there lies your answer.
Google does not penalize .us names. Quite to the contrary, they receive a geotargeted BOOST amongst US searchers. Develop one right, it will rank as well as the same keyword in .com.
For the original posting go to: http://forum.dotsauce.com/24052/are-us-domains-undervalued/