[smtddr]

>>A worm or trojan whose payload steals people's bitcoins is also plausible, though afaik there haven't been any yet.

There have been a couple of these actually:

http://www.pcworld.com/article/2109000/bitcoinstealing-malwa...

http://www.computerworld.com/s/article/9244772/Bitcoin_marke...

http://www.coindesk.com/cointhief-mac-malware-steals-bitcoin...

And the most famous probably, cryptolocker - http://en.wikipedia.org/wiki/CryptoLocker ...though this is more a ransom strategy than just out-right theft.