I have seen wide ranging estimates of this, and I think it's a very important question to study the bitcoin economy. A few public cases have been reported of large sums of bitcoin lost due to hardware malfunction, lost keys, etc . I assume a good chuck of the coins from the early days of mining have been lost. For instance when you look up the first 1000+ blocks a lot of the 50btc that were mined are still in the same addresses (most of them are different)  , I would assume these are lost ? unless you think they are  just dormant waiting on Satoshi to awaken hi master wallet ....