Realistic attack on Bitcoin: High fee spam transactions

Say 1000 small transactions fit per block (for the sake of argument)
If you can expend $20,000 per 10 minutes ($2.8mn per day) you can set the price floor on bitcoin transactions at $20.
For $14mn/day you can put it up to $100.

Solution: Rank transactions based on not only fee-per-byte but also coin-days-destroyed so that shuffling money from one pocket to the other (i.e. spam) is more expensive than ordinary usage.