What if an alternate currency had a large reserve of coins which would be used for a giant coin mixing service for that currency ?
For example opencoin and their 80 billion spare coins could accept "tainted" ripples and swap them out for fresh coins.
Its probably not going to happen for obvious reasons

Just had a similar idea today! Except the currency I offer would be "VGC" -- for virgin coins. Pay the miners a premium for mined coins, then stash them and have a currency back them up. When someone needs to be super secret about a purchase, simply redeem virgin coins.
I thought of it after finding out blockchain.info shut down their coin mixing service. Theres not many mixing services Im aware of that are large enough to offer the same level of mixing that these large sites do.