Apple sensibly stipulate minimum requirements to preserve an adequate user experience.
If you take a cynical viewpoint of that, you could say that perhaps Apple were overly lazy developing Aperture when they stick such high req's on it. If that's true, then it wasn't very sensible ;-) But if that's the case or not we will never know. Either way from what I've seen of Aperture I can understand why it likes a lot of horsepower, but I also think it could run on less than what Apple says. And I think it's perfectly fair to compare minimum requirements.
I ran Lightroom on my mini (1.42GHz, 1Gb RAM) yesterday and it was okay. Obviously I cannot compare to Aperture on the same machine but I imported a bunch of RAW shots from my Nikon D70 and once the import was done (which was rather slow) it's performance was okay. I was impressed, I must say by Lightroom, and I think it'll be a good competitor to Aperture once it's released in a final version. And competition is good, I think ;-)