The statement "What the OS X installer does isn't check how fast your processor is running" is incorrect. I had to comment out the check to get it to install on a G4 with a Sonnet 1.4 GHz processor that was being reported as 0 MHz.

This process may help you: 

http://forums.macrumors.com/showthread.php?t=371302

Leopard will make use of CoreImage ( those Animations like in Tigers' Dashboard "Wellness Effect" ) throughout the complete OS - with Coverflow in the Finder and QuickLook everywhere.

So.. basically to say :

Every MAc OS X edition, way back from 10.1 to Tiger is almost the same, it just needs a lot of Ram and a CPU at about 233-400 MhZ to operate quite well .. WITHOUT those fancy gimicks.. 

And this is the point where the "cut" is made - they simply rule out all macs that have not the graphics adapter required for running CoreImage.

For this it needs a gpu like nvidias 5200 .. well .. it is obvious that Leopard will require a card the same that Windows Vista needs for "Aero", that is - a gpu that has Vertex and Pixel Shader capabilities - to categorize it : a DirectX9-Class Graphics card ... that is a lot of macs that didn't come with such - see, my old Powermac G4 shipped with a built in Ati Rage card which was great back in 2000 but now it sucks.

As this G4 has a pci slot I could upgrade it to the needed card ( there are a lot pci-cards still available ) but what with Emac/imac/ibooks that have NON-REPLACEABLE graphics ?!

see it.. that is why and how the cut is made.. of course Leopard will run even on 8MB graphics - but in slow-motion,.. the OSX desktop is today going to turn into a full accelerated 3d thing - and it seems Apple has no intention to offer some feature like Windows "basic mode" when the setup detects you have not the required card.

this could get dangerous for some people :

Beware : if you run an app that uses CoreImage your gpu will go HOT .. in cases where people tricked imovie o8 to run on non-G5 systems and without a core-image card - some ibooks have DIED.

take care - and don't bother.. Tiger is a Great OS, still after 2 years!

Getting narked at Apple for this seems a tad disingenuous to me - tell me, pray, just how many legacy systems in the 'doze world can run Vista *and* an actual application?

is there any word on a PC compatible version yet?

Never owned a Mac, might go on Ebay and get a cheap one, just to ty this stuff out :)

I can only say that Apple goes downwards.
Apple are iDiots.