Far from perfect, but works:

You deposit bitcoins into your online wallet hosted on a reputable site, you only keep small amounts here for quick transactions like buying gas.
When you buy your stuff, the intermediary site issues an IOU to the merchant, which will be settled on the same day or something.

You risk only small amounts, merchant risks the last few transactions  if the intermediary goes down, not a big deal. It's similar to how
things are today with credit cards, but still faster (for the merchant) and cheaper.