fixed with subquery

SELECT
  date,
  category,
  COUNT(*) AS c1
FROM (
  SELECT date, parse_agent(agent)['category'] AS category
  FROM access_logs
  WHERE service='blog'
    AND (date='20120424' OR date='20120423')
) x
WHERE category='smartphone'
GROUP BY date,category
ORDER BY date,c1 DESC
LIMIT 10

flatten query

SELECT
  date,
  parse_agent(agent)['category'],
  COUNT(*) AS c1
FROM access_logs
WHERE service='blog'
  AND (date='20120424' OR date='20120423')
  AND parse_agent(agent)['category']='smartphone'
GROUP BY date,parse_agent(agent)['category']
ORDER BY date,c1 DESC
LIMIT 10