Apparently it came back up just in time for the sale to expire about a minute later when I tried entering the coupon code which was still showing on the reloaded listing page after the cart told me it was invalid. Great timing.
Well, hopefully the book I was trying to check out goes back on sale at the same price during whatever the next round of Smashwords sales is. And now I have to go clear some expired coupon freebies from my cart. Good thing I hadn't loaded too many in.
Given the timing of it, I think it probably was a Daylight Savings Time switchover issue, which hopefully SW will account for next time they do this.