The seller would also have to trust bitpay (who it seems shopify are using) to store the coins securely, but that's really just like trusting any other payment provider, and hopefully bitpay have insurance and proper financial controls, unlike many exchanges.
Great to see this happening though - I wonder what the comparison on fees is between accepting bitcoin and credit cards, have the fees just moved from one provider (mastercard say) to another (bitpay), or are they significantly lower?
Is there anyone here accepting bitcoin who can enlighten us on the pros and cons?