There is a major unmentioned problem - liquidity. The market price of the ETF/bond would only truly track the price of bitcoin if the two were convertible as a regular occurance. This is the role of an ETF Authorised Participant http://www.investopedia.com/terms/a/authorizedparticipant.asp#axzz1hhYB1nQP - ie. the AP would take dollars and buy bitcoins, then issue a Bitcoin ETF unit to the market. Bond ETF APs do this once a week or so, liquidity is an issue there too. But if there is insufficient liquidity, a "large" transaction isn't possible or sensible. So the market price of the ETF would detach from the market price of bitcoins.

For example, imagine an investor has $10mm USD for investing in bitcoin, and BTCUSD is $4. He will want at least 2mm BTC. I don't think that would be possible even over 1 week. The price would shoot up before even a small part of the order was filled. Any big investor will know this, so they're not interested - they don't want to give everyone else a free lunch, and anything less than $10mm isn't worth the time of day.

Before listing an ETF, we need the BTC market cap to be back at $200mm, and daily trading of $10mm.

What about creating shares in a private company, and trading on SecondMarket.com?