Interesting. This thread on reddit states that it IS an issue in the reference implementation and includes a patch

http://www.reddit.com/r/Bitcoin/comments/1xm49o/due_to_activ...

Edit: The concern with the Bitstamp appears to be confusion due to transaction malleability and NOT actual double spend or cancellation issues. However the BTC reference client is not perfect at handling malleability and only gets it right eventually.