What do you mean by hardware incompatibility? It is their software. If they wanted, they could have spent more time and make it run on existing hardware. Lumia 900 is less than 4 months old. The least they could have done is supported the best (premium) phone running WP at the moment. And who knows, one year down the line there will be WP9 which they will again call incompatible with the existing phone hardware at that time.
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Jelly bean is most definitely 4.1. It's not going to be a major release though.