If by public transport you mean planes, trains and buses for long trips, then yes I have, and will any time I can. There have been numerous occasions where a good diaper has come in handy, though once I over-estimated what it could hold, and well, the seat was soaked when I stood up. Thank goodness I had taken off my suit jacket, or it would have gotten wet too, because I would have been sort of sitting on the lowest part if I had been wearing it. I quickly slipped it on to cover what I could, and grabbed a diaper from my briefcase (just realized how funny THAT is!!!) and holding it in my jacket against me, dashed to the bathroom for a quick change. Some people saw everything, and boy did I get some funny looks!
Another time that wearing a diaper caused some issues, was when I was taking a flight, and decided to wear a cloth diaper and plastic pants. It fit under my clothes pretty well, so I thought why not, who's going to notice? I was not even sure it I would wet in it, thinking that I could always unpin it and pee in the toilet if I wanted to. The thing I had not thought about though, was security. Sure enough, when the guard waved the wand around my waist, there were all those "metal detected" noises. He then said he had to feel in the waist of my pants as they often do even without an alarm, and sure enough, felt all the bulk, and the pins too. He asked my what all this was, and through what felt like burning red cheeks, I told him "I'm wearing a diaper, and those are the pins." I showed him the top edge of my plastic pants, and (sigh of relief) he said "okay, go ahead." This was in the year or so after 9-11, so everyone was still pretty worked up about what might be lurking under peoples clothes. He never expected what I was sneaking on to the plane though - pinned on cotton encased in a plastic panty.