This is closely related to something I was talking about recently:
https://bitcointalk.org/index.php?topic=42514.msg579016#msg579016I absolutely agree that the average hold time strongly affects the fundamental price floor.
However, I think that this change on SR isn't going to cause a market shakeup. Right now the price is divorced from fundamentals, and it's driven mostly by speculation. I'm also not sure that SR is that large a percentage of BTC commerce: I suspect their average transaction is pretty small. Are they moving that much volume? Donations (particularly WikiLeaks) are another "killer app" for Bitcoin, and there are a bunch of IT guys moonlighting for BTC as well. That could move a lot of coins.
That said, the SR hedge feature may matter in a way you overlooked: SR probably hedges the escrowed funds - either by selling short on Bitcoinica to hedge their net position, or directly selling the escrowed coins and buying them back to deliver to the merchant. (Hopefully they've considered the privacy implications.) Do you know if they're doing something like that? If so, the net hold time didn't just go from 8 days to 7, it went from 8 days to probably less than 1.