Nothing will be an iPhone killer for a while. The iPhone killer will be whoever gets themself well establish on a multi-tude of devices.. so there's two ways you can think of an iPhone killer..

hardware = maybe the n900 already killed the iPhone in this aspect. But then the n900 killer would simply be whatever hardware > X

operating system = nobody is going kill the iPhone anytime soon, the operating system will need to be established on several devices.

But your not going see something like what Apple does (one OS limited to one system). Partially because only Apple is really good at it (and to a limited extend Blackberry and Pre). But also because in the long run your cutting yourself off from the rest of the market.