The only A9 cortex that's been announced is the OMAP4 and that still, correct me if I'm wrong, hasn't taped out yet. It's been delayed and TI announced back in July if yields are good, they hope to have volume shipping by 2H2010. Given the amount of time required to develop a device, this means that we may not see a OMAP4 handset until 2011, and the first one for all we know could be an iphone (not that there is anything wrong with that). The next generation beyond snapdragon and omap3 could be further than you think. Other hardware changes could prevent backward compatibility with the n900; all that's needed is an unsupported driver. I would think that it would be in Nokia's best interest to use the n900 as a testbed for maemo6 so the OS is mature by the time they launch the new platform, rather than launch a novel OS on novel hardware.