Buyer spends 1 BTC @ $800 USD/BTC. Value of BTC goes down to $400 USD/BTC. Seller left with 1 BTC worth $400 (if they don't cash out). Buyer requests refund of value @ $800. Seller has 1 BTC @ $400.

I guess this would be mitigated by immediate exchanges... Haven't used Coinbase enough to know how well this would work, specifically their merchant services, but what's the per-transaction cost to immediately exchange BTC on transactions? Also, I've seen buy/sell prices on CB - is that the same in their merchant accounts? I see daily cashouts as a merchant option, but that doesn't entirely remove the fear of a market crash. Or even smaller daily variations (morning price high, evening price low, etc).

I could see how there would be at least some potential for long-term concern.