There is not a 51% attack on bitcoin. There was a pool who was getting a little too close to 51% of the hashing power distribution, but that is very different from a 51% attack. In light of it, major players have committed to switching to P2Pool. Bitcoin is fine.
What exchanges that handle cryptocurrency->fiat transactions does darkcoin not already exist on? We have bitfinex, c-cex, and bter. Additionally, we are on the list at cryptsy for whenever they start allowing coin->fiat transactions.
The legal hurdles in place preclude all but the biggest from even having the option to do so, at least legally.