How much bitcoin is 'lost', on average? As the title of this question states, how much bitcoin is 'lost' (the private keys are not stored by any individual, and so will theoretically never be spent again)? I'd be looking for a percentage per year, ideally. Are there any stats on this anywhere? 

Does it matter that Bitcoin is a deflationary currency, since bitcoins can be lost but cannot created again? It seems like this might encourage hoarding.

Does it vary much from other coins? Is if very different from the amount of fiat currency lost or destroyed unintentionally?
You might want to check out this great comment by Pieter Wuille:


  There will never be more than 20999839.77085749 BTC.
  
  Reasons for this number being lower than the often-cited "21 million":
  
  
  The subsidy scheme (50 BTC per block, halves every 210000 blocks) is rounded down to satoshi's (0.00000001 BTC), which results in slightly
  less already (20999999.9769 to be precise).
  Due to an obscure rule in the original software (which is hard to change now), the 50 BTC generated in the Genesis block are actually
  unspendable, so only 20999949.9769 are left.
  There have been two occurrences (blocks #91842 and #91880) of duplicate coinbases, where the coinbase transaction of a block had the
  same hash as the coinbase of an earlier block, causes the generated
  coins to be "overwritten" in the database. This was made impossible
  later using BIP30 and BIP34, but the coins of those blocks are
  permanently lost (two times 50 BTC).
  Some times, miners have not claimed all fees/subsidy they could in a block, for example block #124724 only claimed 49.99999999 BTC subsidy
  instead of 50.01.
  Soon, with the introduction of provably-unspendable outputs (OP_RETURN), some will not be considered part of the set of spendable
  coins anymore (a total of 0.00835434 BTC currently).
  


Similar topics:


How Many Bitcoins Are Missing?
Bitcoin disappearence rate
Why doesn&#39;t Bitcoin return lost coins back into the block reward?
What could be the consequences of many bitcoins being lost out of circulation if people lose their wallets?
https://bitcointalk.org/index.php?topic=109117.0