Date        ID      wt      value
  01/01/2012  100     0.50    60
  01/01/2012  101     0.75
  01/01/2012  102     1.00    100
  01/02/2012  201     0.50
  01/02/2012  202     1.00    80

select date,id, sum(case when value is not null then value*wt else 0
 end)/nullif(sum(case when value> 0 then wt else 0 end),0) as wt_avg
 from table
 group by date, id