MySQLのSTR_TO_DATE()
関数
SELECT id, date
FROM tableName
WHERE STR_TO_DATE(date,'%Y-%m-%d') >= '01/01/2012'
ORDER BY date DESC;
Unixタイムスタンプを使用する方が簡単だと思いますが
SELECT id, date
FROM tableName
WHERE UNIX_TIMESTAMP(STR_TO_DATE(date,'%d/%m/%Y')) >= UNIX_TIMESTAMP('01/01/2012')
ORDER BY date DESC;