Let's think hard here.  I mean REALLY stretch our brains.  The Winklevoss have spent LOTS of time and money creating an ETF.  Why did they do ths?  Because if this ETF is anything like GLD, it could dramatically increase the price of BTC over a few years.  So the Winklevoss could:
  1. Sell their coins to ETF purchasers, which will leave the BTC exchange rate more or less unchanged and result in the Winklevoss owning no coins.  At that point the BTC exchange rate will start rising but the Winklevoss will not benefit b/c they own no coins.
  2. Keep their coins, forcing ETF purchases to eventually hit the open market pushing up the BTC exchange rate and making their stash much more valuable.

Hmm... which will it be?