Yeah, I guess the biggest issue is someone plugging in john@acmecorp.com just to test the subscription form, and then actual John getting a maillist message and

Yeah, I guess the biggest issue is someone plugging in john@acmecorp.com just to test the subscription form, and then actual John getting a maillist message and tagging it as a spam. The question here is if the same John would or would not tag a plain confirmation message as spam too, and I suspect that he would, in which case what I wrote above is no worse than a double-opt-in in edge cases.