I would say that the most compelling reason to buy a white iPad (at least the reason that I keep coming back to), is the nostalgia behind what we think of when we look at a white apple product. More specifically, I am reminded of when Steve Jobs pulled the first iPod Nano from his coin pocket at the end of the keynote speech which had the Motorola phone with iTunes on it.
It was tiny, it was thinner than a no.2 pencil, it had a colour screen, and it used a sandwich of two materials (white painted acrylic and polished steel) to make it look even thinner. It was a brilliant design and a brilliant political maneuver to boot.
The same (aesthetically) can be said of the two generations of iMac which had articulating screens, as well as that generation's cinema displays. Things that are white/clear give us a sense of the transparent (the un-sensible), and a feeling of lightness with respect to the tasks at hand. It ultimately comes down to the perception (or misperception) that using these products is effortless.
While the black-colored iPads are much more colour-coherent, with respect to the other switches, the logo on the back, and the screen when it is off, there is a sense that it takes itself just a little too seriously. In fact, a black iPad does a better job of being light and transparent than a white one, but doesn't communicate it (a visual contradiction, I know). Don't get me wrong, I believe that all Apple products are seriously well-designed. But as for consumer behaviour, I think that we as consumers love the idea of a product that just works, that is intuitive, that is convenient, that never fails, AND IS FUN.
To that end I believe the answer in color choice depends on your level of reality in product commitment. The newest Apple products are always the funnest are they not? Anyone looking to own this iPad until the next one comes along will probably want a white one (but should keep in mind that anyone wanting to buy second-hand is likely looking for a longer, more practical commitment and probably wants a black one), and anyone looking to own one for longer than a year will probably want a black one. Black ones will have better resale IMO. Only time will tell.
Which brings me to my semi-final point: a White iPad is actually a symbol of your ability to spend money freely, independent of time. So anyone trying make a social statement about their daily economic worth would naturally choose a white one. Many people know this, and will choose a black one out of disgust for those who would choose white, because it's safer to look like you are making a frivolous purchase for 'practical' reasons than to mindlessly jump on a colour bandwagon.
There are also those who would choose white purely because they like it and don't care what other people think. But that's just a game of statistics: if enough people think white is awesome, then enough people will buy them, and nobody has to worry about what it means, because at that point it won't mean anything.
So really the safest answer is that anyone looking to buy right-away should go black. And anyone thinking about white should WAIT AND SEE what everyone else does to make a decision.
Too much analysis? Don't worry, I think so too. But clearly this is what forums are for.