I think the delays have a lot to do with a planned migration to a multitouch UI. It should be evident to anyone who has closely studied Mac's product development cycles that Apple's iPhone is testing the waters of a very powerful type of user interface which allows gestures, sometimes referred to as "chording".
Think of the pre-crime computer in "Minority Report" minus the holography and gloves. That is where I believe Mac and Mac OS are headed very soon. Leopard may be the first significant step in the direction of EXACTLY that type of user interface. Jeff Han of NYU demonstrated this quite clearly at the TED conference with his live demonstration on large multitouch user interfaces that has been doing the rounds on YouTube.
At any rate, I've spoken with a few engineers, one of whom used to work for Apple, and they're not surprised that Apple may be testing prototypes of multitouch iMacs as we speak.
The patents are certainly in place, the UI definitely shows signs of moving toward a more three-dimensional design. It's just a matter of when the product will hit the market.
That announcement *could* come as early as WWDC. It's not at all inconceivable... the multitouch technology is already here. Apple's already releasing one product with it, and they started developing it several years ago... which means by now they're almost certainly testing prototypes of larger scale multitouch interfaces running OS prototypes.
Anyone who thinks they're not thinking this far ahead is easily forgetting how they stunned the world when they revealed, after announcing the Intel Macs, that OS X had already been coded as dual binaries since 10.1.
It certainly would explain a lot of the latency and secrecy around Leopard... and it fits with their product strategy.
Besides, Guy Kawasaki once said among their many other strengths, their strategic advantage is their focus industrial design... on just making "cool stuff" that people want to use. So, if the reason is as simple as "Multitouch, because we can"... Apple's there, and there's an ulterior motive that Steve Jobs has been gunning for ever since he came back on board. They could be preparing to leverage the Multitouch Mac as a deathblow in response to Microsoft's feeble attempt to catch up with Vista.