Ok, the new ones are crypt MD5 salted hashes. But 2 months ago they were all plain MD5 unsalted.
Let me try to go more descriptive here, to see if you understand my line of deduction:
Notice there the evolution in the prices 2 months ago:
My theory goes around that
by such time MtGox's db was already compromised.
At this point the hacker would start to get pwds and entering some accounts to check its users' activity. Allowing him to sort some dormant accounts with credit. Those with BTC on it were probably just wiped out, those with USD the hacker used it to buy BTC (bringing the price up this way) and later cashed out the BTC resulting from those trades.
Some of those users started to complaint to M'Tux who decided to implement his crypt(pass,salt) function. Up to this point, and up to yesterday M'Tux was most likely believing it was about users' giving away their passwords not his site fault.
Lately the thief tripped some wires or the legit owners checked their accounts making a load of requests to MtGox who came forward and put that thread on the forum. Seeing this, the robber decides to go on his final blow... and here we are.