Currently about 20000BTC are stuck in failed transactions from Gox, that nearly matches the remaining BTCleft on Mt. Gox askside.
[tinfoil hat on] Maybe they are already out of coins.[tinfoil hat off]

Look here about the missing withdrawls:
http://thegoxreport.com/

and the remaining asks on Gox
http://trading.i286.org/



BTW: The daily missing tx (18770)and the remaining(18870) match each other nearly exactly