http://search.twitter.com/search.atom?ands=olympics+covergirl&rpp=100 contains results from the 19, and the 20th of Feb.
However, http://search.twitter.com/search.atom?ands=olympics+covergirl&rpp=100&since=2010-02-20&until=2010-02-20 still returns tweets from the 19th ? is this a timezone issue ? can we specify the timezone in which the dates restrictions are specified ?