I just tweeted Cryptsy asking them to disclose their policy on Proof of Stake coins, it would be nice to know that selling pressure isn't coming from them. If they don't want to give stake payments to traders, they could donate coins to a mintcoin fund instead, that would be a simple and elegant solution.
I think that Mintpal needs to provide a disclosure like this as well. For all we know, a process is in place to dump interest from coins directly to the market adding to those sell walls. That's what I like about Prelude by Moolah where they've come out and said they will give back that interest.
I know MintPal has stated they disable PoS function on coins if possible. I think most exchanges do the same.