GreenDog,
I was a MM in SPY's on the AMEX some years back and from my experience, even though ETF's are open-ended, they can be hard to borrow in the short run.
ETF's are share's that represent ownership in a Basket of Stocks, like the NASDAQ 100 or S & P 500 index. The shares are held in a depositary trust at a bank . More ETF's can be created by a deposit of a basket of shares at the trust, called a Creation Unit.
Autthorized Participants are institutional investors, Market Maker's or clearing firms that allows the participants to directly transact business with a depositary trust and create more ETF's .
Since someone must be long a ETF for a short seller to borrow the ETF to "Sell Short", there could be times where ETF's are hard to borrow, unless you can create the ETF's yourself and deposit them at the trust institution!
If market players are bearish or MM's have Sold many QQQ's( for example) Short to buyers, then there could be a short sqeeze in QQQ's in the short run(no pun intended!).
For more information on ETF's , you can go to
www.amex.com/ .
They have some good information on ETF's.
Gene Weissman
E-Brokerage, LLC
gene@ebrk.com