Colin's answer is right, except that I see — in line with avid's comment — that the different temporal focus indeed indicates a small difference in meaning.
When I say "wasn't it supposed to start 30 minutes ago" I talk about the actual start. A good example would be fireworks where the start would be an event in itself, and everybody is waiting for it.
By contrast, with "wasn't the talk show supposed to have started 30 minutes ago" after switching on the TV I'm focusing on the ongoing talk show after it has started, and less on the start proper.
When speaking casually one could use both tenses interchangeably. I also made an effort to use two examples where the difference really mattered (fireworks have a spectacular start, talk shows do not), which is often not the case.