that's interesting about the fungibility, i hadn't heard that. I also just realized that the contract size is only 100 shares, I had previously thought it was 1000 shares for some reason. I am starting to warm up to this whole idea. At first I thought it was totally unecessary, but now I see it as a real alternative to buying stock on margin and paying interest. My concerns now are with the spreads. If the spreads are much larger than the spread on the underlying stock then they won't make much sense for somebody that is getting 4:1 margin or higher on stocks. But if the spreads are similar to the stock spreads, and they are exempt from the PDT, then they may have a real winner.
Oh one more thing, i just read that they can be traded from a regular brokerage account, no futures account required. I guess all you will do is sign a SSF agreement just like an option agreement and then you are off and trading.