Kirk McElhearn over at Macworld has attempted to answer the very first question I've had since I first heard that the iPad wasn't going to be in a widescreen format: Why 4:3? The answer: because for the majority of things you'll be using your iPad for, that's the right resolution you'll want. Magazines, books, the web, productivity, and so on, McElhearn argues that 4:3 is the right way to go.
Of course, taste plays a factor in there -- I prefer to browse the Internet on a widescreen monitor, actually, and I think games, which I'll be playing a lot of on my iPad when I eventually get one, tend to work better with a wider field of view. And let's be honest, the 4:3 ratio only allows Apple to start small and go with an "iPad XL" or an "iPad HD" in the future, widening the screen and perhaps even upping the resolution.
But McElhearn makes a good point that the 4:3 ratio has been Apple's home until only just recently, and for most objects that you hold in your hands and read like the iPad, from magazines to newspapers, the 4:3 ratio still fits. The ratio likely won't affect sales at all, but we'll have to see if movie watchers hold out for a better format for their media.