The new imac won't come out til Ivy Bridge is ready. And even then, there's no guarantee it's a huge advantage over the current sandy bridge. Especially since no one is really CPU bound anymore. Get the computer you need when you need it.
So what you're really saying is the quality and desirability of apple's products result in their holding their value for a much longer period of time than similar products from other companies.
I agree. I think it's because they're methodical with their updates, and pay attention to the details necessary to make a quality product.