Maybe we will see some pirates demanding bitcoins for ransom in the future?
It is funny, but when I first learned about bitcoins and saw its potential, I thought, yeah, it is going to be the way ransoms are demanded in the future.
The problem with ransoms are you either need to be incredibly smart with inter-bank systems so that you can effectively receive money that you know can't be taken back, or you have to physically receive a satchel full of actual cash or gold (a very dangerous part of the whole process).
Now you can deposit your captured person in a location, get your bitcoins sent to you, and then give over the info as to where the person is (the last part is optional).
Add in the coming escrow option, and this gets even more clear for both parties in the exchange.
This works well also for drug trade and a bunch of other illegal operations where working with banking systems are not a viable option.
So far, I think criminals are slowly drifting into bitcoin. The small scale drug dealers are the first (with the silk road). I can tell it is not major yet though, as the price of bitcoins will skyrocket once it becomes the standard of criminal currency. With the potential of only 9 Million currently available to go around, and a price of $5 each, that doesn't leave much to go around for major criminal transactions.