Or Apple may be waiting for the Santa Rosa mobile chipset to be released.
That is April last time I saw anything...
Given that Apple do not always rush to use Intel tech as it is rolled out, it could be some time after that. Would not be surprising to see them release Leopard, and then once the feeding frenzy to buy a Mac with the new OS dies down they then start to update the whole product line (Santa Roras in Mac Mini, iMac, MacBook, 8 Core Mac Pro etc).
Or they could doo it the other way around. Push out updates first, then intrpoduce Leopard. If such is the case then Leopard is going to be later than anticipated (June-July and not April)
Apple is a business - they will want to milk as much ast they can out of their existing products as possible 😀