So what is this?
This could be found at Iovine Brothers Produce Saturday at the Reading Terminal Market. The fruits, about four or five inches long, reminded me of cactus pears (a.k.a. prickly pears). Although one of the Iovine managers told me they are similar in taste (and structure, with tiny seeds mixed in with the flesh) to kiwifruit, some Googling uncovered the fact that these Dragonfruit (more properly called Pitaya) are, indeed, the fruit of a cactus-like plant from semi-arid regions. The flesh can be either red or white. I passed them by, but maybe I'll try them (if I can extract the seeds, which can be eaten but are indigestible) in a sorbet.