Yes, that's true for some products, but the iPad might be in another category, like the Mac Pro and iPods, which are updated on longer cycles.
Apple said last year that the iPad is a new product category and they're still analyzing consumer's behavior regarding that product.
Last week I received a questionaire about my iPad, with many questions trying to understand how I use it, how many people have it in my house, etc.
They'll have to take risks to test the market and reserve momentum for a new size and possibly new features, like USB, so they can update the iPad further next year, or even early next year.