I see some BTC to BTC trades on Ripple. Are these just transfers from one exchange to another? Or, is Ripple being used as a mixer?
The counter-parties are different.
In Ripple chain: BTC has an issuer, it can be Bitstamp, Snapswap, Gatehub or any other gateway. So if you send your BTC to Bitstamp, they will give you BTC (issued by Bitstamp), if you send BTC to GateHub, you will get BTC (issued by Gatehub).
If you have BTC (SnapSwap) and you want to withdraw BTC from your Bitstamp account, BTC must be exchanged from BTC Snapswap to BTC Bitstamp. Though it can be done automatically (path can be found)