And if Apple surprises us all with a MP in Paris? My guess at this point would be Paris Expo for a new MP. Apple changes the main web page every three weeks or so. And it would be just in time to switch come Paris. So let's just hope they get the stuff early and surprise us. Apple would want everyone to buy Leopard anyway rather than getting it for free on a new system.
The biggest argument against this is Apple not wanting to write drivers for Tiger and Leopard for several new GPUs.