I can lose $20 right now and it would be impossible to trace back. Bitcoin really isn't any different than cash, except bitcoin users probably understand the risks a lot more than $random_consumer.
I don't want to bag on the guy who wrote Bitcoinica, except that I wish he had done what I said earlier and built a play-money exchange instead of a "real"†-money system like this.
But I'm happy to berate the people who talk up Bitcoin, the Bitcoin economy, and the ecosystem of Bitcoin services and then, when things fail to the tune of 6-figure losses for customers, try to apologize around it by saying "oh, well, that service was built by a teenager; wait until the serious engineers get around to building Bitcoin services!". It's a frustrating and intellectually dishonest argument to make.
† Heh
Look, at the end of the day you're hearing only the exception not the norm. Its like people who watch the nightly news and think their little suburb is full of terrorist and murderers. Confirmation bias is a hellava drug.