I think Leopard is supposed to run natively on both flavors of Mac.I never upgraded to Tiger, and since I don't do anything more challenging on this Mac than a trivial spreadsheet, I don't actually need a shiny new computer or even a new OS. But I think at some point ya hafta find out what the noise is about . . .
Off topic a bit, Walter Mossberg's advice column in the WSJ today quotes Steve Jobs to the effect that the iPhone will not be capable of accepting third-party software out of the box on June 29 - but that "that is our intent," and he expects it to happen by the end of the year.
A pundit as unlikely as Arianna Huffington sought out Mr. Jobs directly for advice on being the first to score a phone. (He told her to go to an AT&T store.)*grin*